They are vertical angles, and so set them equal to each other
x + 40 = 3x
Isolate the x. Subtract x from both sides
x (-x) + 40 = 3x (-x)
Simplify
40 = 3x - x
40 = 2x
Isolate the x. Divide 2 from both sides
40/2 = 2x/2
x = 40/2
x = 20
20, or (A) is your answer
hope this helps
