1. x^2 + y^2 = 225
y^2 = -x^2 + 225
y = +_SQRT(-x^2 + 225)
Ellipse
It could be .... I really don't know since you haven't said waht the original question was.
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.