this is a linear system 
you can graph both lines and see where the intercept or make an equation for both and then solve it for a point that can solve both equations. I'm going to use the subsitution method 
x is steak 
y is chicken 
x+y=6 <-- for the amount of steaks + amount of chicken should be equal to 6 meals, 1 for each person 
17x+14.80y=91 <--$17 for each steak + 14.80 for each chicken should be $91 
x+y=6 
get one of the 2 variables alone on one side. I'm doing it for x 
x=-y+6 
since thats what y is in terms of x, subsitute that for x in the other equation 
17(-y+6)+14.80y=91 
distribute the 17 
-17y+102+14.80y=91 
combine like terms 
-2.2y+102=91 
subtract 102 
-2.2y=-11 
divide by 102 now you have y alone 
y=5 
because before you got y in terms of x you know that x is equivalent to negative y + 6. now we have y (5) so subsitute that in to get x 
x=-(5)+6 
simplify 
x=1 
now you have x=1 and y=5. you know y is chicken and x is steak so theres your answer! (1 person got steak 5 people got chicken) 
you can plug in x and y in both quations and make sure its right 
x+y=6 --> 1+5=6 --> 6=6 
that eqaution's true 
17x+14.80y=91 --> 17(1)+14.80(5)=91 --> 17+74=91 --> 91=91 
both equations are true so your answer is right 
hope that hhelps you :)