How to do this ? i don't understand how to do this

Ok, so on the top, the angle is 100 degrees on a line, a line is a 180 degree angle so the angle to the left of that top angle is 80 degrees, 80 - 10 would give you the answer for x because (x + 10) is equal to the angle to the left of the 100 degree angle

Answerprediction;