Answer:
x=59
Explanation:
The sum of the interior angles of a triangle add up to 180 degrees. That means that (x+12)+(x)+(x-9)=180 degrees in total. If we combine the like terms (combine the xs and regular constant values), the xs add up to 3x (x+x+x) and the regular constants add up to 3 (12+-9=3). If we rewrite this in a more simplified form, we get 3x+3=180. Subtract 3 on both sides to get 3x=177 because we want to isolate x. 3x=177 divide 3 on both sides and we get 59. x=59