We know this angle is a 90° or a right angle. So another words we know adding these 2 expressions equals to 90°.
(4x + 3) + (x - 8) = 90°
Remove the parentheses and combine the like-terms:
5x - 5 = 90°
Add 5 on both sides:
5x = 95°
Divide 5 on both sides
x = 19
You answer is x = 19.