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
