if $x= 2\cos\theta - \cos 2\theta$ and $y = 2\sin \theta -\sin 2\theta$ find $d^2y/dx^2$ at $\theta = \pi/2$

Question

if $$ \begin{cases} x= 2 \cos \theta - \cos 2\theta \\ y = 2 \sin \theta - \sin 2\theta \end{cases} $$ find $d^2y/dx^2$ at $\theta = \frac{\pi}{2}$

I have solved $\frac{dy}{d\theta}$ and $\frac{dx}{d\theta}$ and from this also solved $$ \frac{dy}{dx}= \frac{\cos {\theta}-\cos{ 2\theta}}{-\sin {\theta} +\sin{2\theta}}$$

after this I am confused.