If the problem is seeking a numeric answer then two equations would be required.  A linear equation system can be solved numerically, a non-linear system would be solved graphically. 

The solver first manipulates an equation into its standard form before graphing; for a linear equation, that form is "y=mx+b" where the slope "m" may then be determined by inspecting the equation (for example, in the equation "y=-2x+4" "-2" is the correct value for "m"). For a parabolic equation, the standard form is y = a(x - h)^2 + k, from which direction (polarity of "a") and axis of symmetry (value of "h"), etc. may be determined by inspection. Your equation is a circle and is expressed in the standard form.