alpha = PI - 5/8 PI
alpha = 3/8 PI
The angle 5/8 Pi is found in the second quadrant.
The reference angle for any angle (A) in the second quadrant is given by

Therefore, we can solve for 5/8 Pi:

But in order to put it in the box in the question:

alpha = PI - 5/8 PI
alpha = 3/8 PI