Question is on an assignment for polar curves. I am sure that at vertical turning point dx/dtheta=0 will be used. I'm not sure what sign means, could we just presume its a coefficient? It's definitely not sin.

So far I have tried d2x/dy2=d/dy(dx/dy)=d/dy(dx/dtheta*dtheta/dy) but I don't know how to go from there.

Any help appreciated.