How do i determine the radius of x^2+y^2=25 ?

The general formula is:
x^2 + y^2 = r^2
where x and y form the circle while r is the radius.

You already have the problem set up in its general form and you don't want the radius squared... so square root everything.

If you do that then you'll get:
x + y = 5
where 5 is the radius and NOT the radius squared. If you have questions then leave a comment. Good luck!