Answer:
x = 10
Step-by-step explanation:
The exterior angle of a triangle is equal to the sum of the 2 opposite interior angles.
12x + 15 is an exterior angle of the triangle , then
12x + 15 = 4x + 10 + 85 , that is
12x + 15 = 4x + 95 ( subtract 4x from both sides )
8x + 15 = 95 ( subtract 15 from both sides )
8x = 80 ( divide both sides by 8 )
x = 10
