[tex] R = X^{2} + Y^{2} [/tex]
[tex] Y^{2} = R - X^{2} [/tex]
[tex] Y = \sqrt{R - X^{2} } [/tex]
[tex] R=X^2 + Y^2\\Y^2=R-X^2\\Y=\sqrt{R-X^2} [/tex]
That, assuming all the variables are non-negative numbers.
Otherwise, there's one more soultion:
[tex] Y=-\sqrt{R-X^2} [/tex]
