Calculus/trig

The wavelength here is    $\pi$   and that is the only transformation from the parent function of   y=cos x

If y= cos nx   then the wavelength will be     $\frac{2\pi}{n}$

so     

$\frac{2\pi}{n}=\pi\\ \frac{2}{n}=1\\ n=2$

So it must be      $y=cos(2x)$

Check by using Desmos graphing calculator.

Desmos is another tool that is very easy to use and very helpful to you. 

https://www.desmos.com/calculator/t7prurevsn