Answer
$$ y = 2 \cos \left(\frac{1}{3} x-\frac{\pi}{4}\right)$$
Work Step by Step
We are given:
$$f(x)=\cos x$$
Then the composite functions is:
\begin{align*}
y&=2 f\left(\frac{1}{3} x-\frac{\pi}{4}\right)\\
&= 2 \cos \left(\frac{1}{3} x-\frac{\pi}{4}\right)
\end{align*}
We sketch the result below.
