Which explains how to find the radius of a circle whose equation is in the form x2 y2 = z?

If the expression is x^2 + y^2 = z, means that the radius is √z.

That is because the general expression of a circle with the center at the origin of the coordinate system is x^2 + y^2 = (radius) ^2

Then in your expression z = (radius) ^2, and the radius is found by taking square root>

√ (radius) ^2 = √z

=>radius = √z