The angles of a quadrilateral add to 360, so we can solve for x by adding the 4 angle measures together and setting it equal to 360:
90 + 140 + (x - 10) + (x - 20) = 360
Combine like terms:
230 + 2x - 30 = 360
200 + 2x = 360
200 - (200) + 2x = 360 - (200)
2x = 160
Divide both sides by 2:
2x/(2) = 160/(2)
x = 80