Respuesta :
Answers:
  1)  The value of AB  is:  " 59 units."
  2) The value of BC  is:  "21 units."
_______
  3) The value of AB  +  BC  ;
= 59 units + 21 units ;
= [the value of AC ] ;
      =   {" 80 units ".}.
_______
Step-by-step explanation:
 Given the following—and based on the assumption that we are dealing with:  "geometric lines"—and/or: "geometric line segments"—we are asked to:  
   "Find the value of:  " AB and BC" .
     {Given:  " AB = (8x + 3) ;  BC  =  (2x + 7) ;  AC = 80 ".}.
 ----------------------------------------------------------------------------------------------=-
Note:  The following "line graphs" are "Not Drawn to Scale."
| (8x + 3) | (2x + 7) |
|<--------------->|<------------------->|
 <----·|----------------->|<------------------->|  {Note:  "Not drawn to scale."}.
        A                   B                         C
 <-----|----------------->|<------------------->|
A C
<-----|<--------------------------------------->|
<-----|----------------------------------------->|
A {"AC = 80 ".} C
  <---|------------------------------------------>|
        A       {"AC = 80 ".}.                   C
| (8x + 3) ; + (2x + 7) = 80 ;
<-----|----------------->|<--------------------->I
        A                    B                         C
So:  We have to find the value of:
      1)  AB ; which is: (8x + 3) ;  And:
      2) BC ;  which is:  (2x + 7) .
To get these values; we need to find the value for "x".
So:   (8x + 3)  +  (2x + 7)   =  80  ;  
       ➝  8x + 3 + 2x + 7  =  80 ; 
       ➝ Now, Let us combine the "like terms" on the "left-hand side" of the equation:
     +8x + 2x + 3 + 7  ;
       ➝  to get: + 8x + 2x = + 10x  ; and:
                        +3 + 7 = 10 ;
To get: "10x + 10" ;
 Now, we can rewrite the equation:
        {" 70 ÷ 10 = 7 ."}.   " 10x + 10 = 80 ; 
_____________________
To solve for "x" ; there are many ways:
_____________________
Method 1)  We have:  " 10x + 10 = 80 " ; 
  ➝ Now, subtract "10" from each side of the question; 
        10x + 10 − 10 = 80 − 10 ; 
 to get:  10x = 70 ;
➝ Now, divide each side of the equation by: "10" ; to isolate "x" on one side of the equation; & to solve for "x" ;
10x /10 = 70 /10 ; to get: " x = 7 "
Method 2)
  At the moment above in which we have: 
 " 10x = 70 " ;   we know that "10x ÷ 10 = 1x" ; and then "70 ÷ 10 = 7 ".
        {Note that any value; divided by "10" ;  is equal to: 
 {"that value" moved Back by:  "One" decimal space.}.  
So: " 1x = 7 " ; ↔ " x = 7 ".
_____________
Method 3) : 
When we have:  " 10x /10 = 70 /10 " ;  we have " 1x " on the "left-hand side" of the equation;  and:  "{ 70 /10 }" = {" 70 ÷ 10 = 7 ."}.
Note that:  {" 70 ÷ 10 = 7 "} ; can be determined in many ways;
     For instance:  {" 70 ÷ 10 =  ? "} ;  
➝ The zeros for Both the numerator and denominator "cancel out"—
and we have:  " [tex]\frac{70}{10}[/tex] " ;  
  ➝  Cancel out each of the two (2) "zeros" ; and we have:  " [tex]\frac{7}{1} = 7.[/tex] "
➝ This is assuming that we figure out that: " [tex]\frac{10x}{10}=1x =x[/tex] ."
___________
Now; let's find the correct values for this Brainly Question:
 1) AB  =  8x + 3 ;  
 Substitute our calculated value for "x" ; and solve:
➝ AB = 8x + 3 ; ↔ 8(7) + 3 = 56 + 3 = 59 .
 ➝  BC  = 2x + 7 ;  ↔  2(7)  +  7   =  14  + 7   =  21 .
Note:
 ➝  AC  =  80 = AB  +  BC   ≟   59 + 21  ≟   80 ?  Yes!
____________
Answers:
  1)  The value of AB  is:  " 59 units."
  ___
  2) The value of BC  is:   "21 units."
___
3) The value of AB + BC ;
= 59 units + 21 units = {The value of AC } = "80 units".}.
_____
Hope this answer—along with explanations—is helpful to you. 
Best wishes in your academic pursuits!
______
