Because it is an isosceles trapezoid, angle B and C should be equal. So you would set B and C equal to each other. So: x+10=2x-30. Get x on one side, so you subtract x from 2x and get: 10=x-30. Now you need to get x by itself, so you add 30 to each side and get: 10+30=x. So add 10 and 30: and x=40.