If $x=a\:cos\theta+b\:sin\theta$ and $y=a\:sin\theta-b\:cos\theta$, then show that $y^2.\large\frac{d^2y}{dx^2}$$-x\large\frac{dy}{dx}$$+y=0$

Question

$(a)\;1\qquad(b)\;0\qquad(c)\;2\qquad(d)\;4$