Answer
8; 16
Work Step by Step
We know that if $A$ is a real number and $B\gt0$, then the graphs of $y=Asin(Bx)$ and $y=Acos(Bx)$ will have an amplitude of $|A|$ and a period of $\frac{2\pi}{B}$.
Therefore, the amplitude of the graph of $y=-8cos\frac{\pi}{8}x$ is $|-8|=8$
Therefore, the period of the graph of $y=-8cos\frac{\pi}{8}x$ is $\frac{2\pi}{\frac{\pi}{8}}=2\pi\times\frac{8}{\pi}=\frac{16\pi}{\pi}=\frac{16\pi\div\pi}{\pi\div\pi}=16$.