x^2 + y^2 = r^2 so you get the units of the x at the x axis, square it, and add the y axis units and then, use it like a^2 +b^2 = c^2 (c^2 being the new r^2 radius of the circle). After getting r^2, just square root it to get rid of the ^2, and there you have your answer :>
Hope this helps :)
