I always understood the unit circle to have a radius of 1 and be centered on the origin (0,0). That there are two x coordinates confuses me. Neither 8 nor 17 is an x value for a line passing through the unit circle, so I'm going to guess "8/17" is the actual x coordinate (which is between -1 and 1). P(8/17,y) = x^2 + y^2 = 1.
289/289 - 64/289 = 225/289. The square root of that (positive since y is positive) yields 15/17. P(x,y) = 8/17, 15/17.
