Respuesta :

To find the measures  A and C you first have to know that all the angles of a triangle sum up to 180.

12x+12+3x+18+15=180
(simplify)
(12x+3x)+(12+18+15)=180
(15x)+(45)=180
then subtract 45 from both sides 
15x+45-45=180-45

15x=135 
then divide both sides by 15 
 [tex] \frac{15x}{15} [/tex] = [tex] \frac{135}{15} [/tex]

x=9

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