Answer
$y = -cos~2x$
We can see the graph below.
Work Step by Step
$y = -cos~2x$
When $x = 0$, then $y = -cos~0 = -1$
When $x = \frac{\pi}{4}$, then $y = -cos~\frac{\pi}{2} = 0$
When $x = \frac{\pi}{2}$, then $y = -cos~\pi= 1$
When $x = \frac{3\pi}{4}$, then $y = -cos~\frac{3\pi}{2} = 0$
When $x = \pi$, then $y = -cos~2\pi = -1$
We can see the graph below.