It's a complimentary angle.
Complimentary angles both add up to 90 degrees, as you can also tell by the square with indicates a 90 degree angle.
Make (x + 54) + 5x = 90.
x + 54 + 5x = 90
Add like terms.
6x + 54 = 90
Subtract the 54.
6x = 36
Divide by 6.
x = 6
