They are vertical angles.
To solve for x, remember that vertical angles are equal to one another.
So (10x - 15) = (3x + 20)
+15 +15
10x = 3x + 35
-3x -3x
7x = 35; divide both sides by 7 to isolate x and you get x = 5
Plug 7 in for x in for one equation, and the angle equals 35 degrees. This is true for both angles because they are vertical angles
