The top angle (10x+10) is equal to the angle below the lowest line and the skewed line. However, the 4x+2 is on the other side. But both together add up to 180° (a straight line).
So 10x+10 = 180-(4x+2) is the formula you're looking for.
Solving it:
10x+10 = 180-(4x+2) =>
10x+10 = 178-4x =>
14x = 168 =>
x = 12
Look up the theory for F angles and Z angles.
