Chapter 6 - The Circular Functions and Their Graphs - 6.3 Graphs of the Sine and Cosine Functions - 6.3 Exercises - Page 602: 41

$y=2\ cos(2x)$

As $f(0)\ne0$, use the form $y=a\ cos(bx)$. We can identify the period as $p=\pi$, thus $\frac{2\pi}{b}=\pi$ and $b=2$. We can find the amplitude as $2$, let $a=2$ which is good for $f(0)=2$. The answer is $y=2\ cos(2x)$