To convert, use y=r sin theta and x=r cos theta
replace all x and y the equivalents
y=y/x-(x+y)^2
r sin theta=(r sin theta)/(r cos theta)-( r cos theta+ r sin theta)^2
r sin theta=(cancelr sin theta)/(cancelr cos theta)-( r^2)( cos theta+ sin theta)^2
r sin theta=(sin theta)/( cos theta)-( r^2)( cos^2 theta+ sin^2 theta+2 sin theta cos theta)
r sin theta= tan theta-r^2(1+sin 2theta)
r^2(1+sin 2theta)+r sin theta-tan theta=0