The equation of the unit circle is
x^2 + y^2 = 1
If x = 5/8, then solving for y
(5/8)^2 + y^2 = 1
y^2 = 1 - 25/64
y^2 = 39/64
y = sqrt(39/64)
We use the positive square root because positive x-coordinate and positive y-coordinate is in the first quadrant.
Therefore the answer to this question is sqrt(39)/8