Determine the sum of the series
$\sum_{n=2}^\infty \frac{(-1)^nn}{(2n)!} x^{2n}$
I realize that there is a sum by comparison for $\cos x$ which is defined by
$\sum_{n=0}^\infty \frac{(-1)^n}{(2n)!} x^{2n} = \cos x$
However, how would I go about converting it to this form as the answer I get does not conform with the posted solution