Answer
The graph of $-cos~3x$ with two periods is shown below.
Work Step by Step
The period of $cos~x$ is $2\pi$. We want two periods ($4\pi$). Then,
$0\lt3x\lt4\pi$
$0\lt x\lt\frac{4\pi}{3}$
Maximum:
$-cos~3x=1$
$cos~3x=-1$
$3x=\pi$ and $3x=3\pi$
$x=\frac{\pi}{3}$ and $x=\pi$
Minimum:
$-cos~3x=-1$
$cos~3x=1$
$3x=0$ and $3x=2\pi$
$x=0$ and $x=\frac{2\pi}{3}$
