Answer:
C
Step-by-step explanation:
The exterior angle of a triangle is equal to the sum of the 2 opposite interior angles.
(7x - 40) is an exterior angle of the triangle, then
7x - 40 = 3x + 3x - 20
7x - 40 = 6x - 20 ( subtract 6x from both sides )
x - 40 = - 20 ( add 40 to both sides )
x = 20 → C
