AHSME 1975 Problem 30

Let $x=\cos 36^{\circ} - \cos 72^{\circ}$. Then $x$ equals $\textbf{(A)}\ \frac{1}{3} \textbf{(B)}\ \frac{1}{2} \textbf{(C)}\ 3-\sqrt{6} \textbf{(D)}\ 2\sqrt{3}-3 \textbf{(E)}\ \text{none of these}$