Note that this is a vertical angle, and so the angles will be congruent. Set the measurements equal to each other, isolate the x, and solve for x:
3x + 20 = 5x - 16
First, subtract 5x from both sides, and 20 from both sides
3x (-5x) + 20 (-20) = 5x (-5x) - 16 (-20)
3x - 5x = -16 - 20
Simplify. Combine like terms
-2x = -36
Isolate the x. Divide -2 from both sides
(-2x)/-2 = (-36)/-2
x = (-36)/-2
x = 18
18 is your answer for x
hope this helps