We are asked to find the Maclaurin series for a function involving cos(x). Recall the Maclaurin series for cos(x).
$$cos(x) = \sum_{n=0}^{\infty} (-1)^n \frac{x^{2n}}{(2n)!}$$
The same equality would be true for any variable, and in particular for $$u = \frac{1}{13}x^2$$.
Therefore, the Maclaurin series for $$cos\left(\frac{1}{13}x^2\right)$$ is
$$\sum_{n=0}^{\infty} (-1)^n \frac{\left(\boxed{\phantom{\frac{1}{13}x^2}}\right)^{2n}}{(2n)!} = \sum_{n=0}^{\infty} (-1)^n \frac{x^{4n}}{\boxed{\phantom{(2n)!}}(2n)!}$$