If $x^2 y-x^3 \large\frac{dy}{dx}$$=y^4 \cos x$ then $x^3\;y$ is equal to :

Question

$$\begin {array} {1 1} (a)\;\sin x & \quad (b)\;2 \sin x+c \\ (c)\;-3 \sin x +c & \quad (d)\;3 \cos x +c \end {array}$$