Answer:
x = 5
Step-by-step explanation:
the exterior angle of a triangle is equal to the sum of the 2 opposite interior angles.
21x + 5 is an exterior angle of the triangle , then
21x + 5 = 5 + 13x + 40
21x + 5 = 13x + 45 ( subtract 13x from both sides )
8x + 5 = 45 ( subtract 5 from both sides )
8x = 40 ( divide both sides by 8 )
x = 5
