Anybody know the answer?

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