As you should already know that the angle shown is 90°.
So,
x + (x - 20) = 90
Now we have to solve for x. To find x we need to isolate it (get it by itself)
Add 20 on both sides:
x + x = 110
Simplify:
2x = 110
Now divide 2 on both sides (to get x by itself)
x = 55
Let’s check our answer:
55 + ( 55 - 20) = 90
55 + 35 = 90
90 = 90
So your answer should be x = 55.
