Answer
$$\cos (3x)$$
Work Step by Step
The Taylor series for $\cos x $ can be defined as: $\cos x=1-\dfrac{ x^2}{2!}+\dfrac{x^4}{4!}-....$
Consider the given series:
$\\=1-\dfrac{3^2 \cdot x^2}{2!}+\dfrac{3^4 \cdot x^4}{4!} - \dfrac{3^6 \cdot x^6}{6!}+.....\\=1-\dfrac{1}{2!}(3x)^2+( \dfrac{1}{4!}) (3x)^4-......\\=\cos (3x)$
