(x-x0)^2+ (y-y0)^2 = R^2$$(x-x0)^{2}+(y-y0)^{2}$$ = $$R^{2}$$
(x-x0)^2 = R^2- (y-y0)^2$$(x-x0)^{2}$$ = $$R^{2}-(y-y0)^{2}$$
(x-x0)^2 = R^2- y^2+ 2* y* y0- y0^2$$(x-x0)^{2}$$ = $$R^{2}-y^{2}+2\cdot y\cdot y0-y0^{2}$$
x = saknis( R^2- y^2+ 2* y* y0- y0^2)+x0$$x$$ = $$\sqrt {R^{2}-y^{2}+2\cdot y\cdot y0-y0^{2}}+x0$$