1. x^2 + y^2 = 225
y^2 = -x^2 + 225
y = +_SQRT(-x^2 + 225)
Maybe your equation x^2 + y^2 = 225 is wrong?
Since you haven't posted the actual question you were working on, nor shown any working, and since my crystal ball is currently being serviced, at this point in time nothing further can be said to help you.