1: Identify the angle.
The following angle is a 90.
2: Set up the equation.
x + (3x + 10) = 90
3: Combine like terms.
4x + 10 = 90
4: Solve for x.
4x + 10 = 90
-10 = - 10
--------------------------
4x = 80
------------------
4 4
x = 20
Your answer is x = 20.
Hope this helps! Have a good day/night! c: