Answer
$$\frac{1}{2}, \\ \frac{2\pi}{3}$$
Work Step by Step
Please note that the graph of $y=A \cos Bx$ has amplitude $|A|$ and period $\frac{2\pi }{B}$. So the amplitude of $y=\frac{1}{2} \cos 3x$ is $|\frac{1}{2}|=\frac{1}{2}$ and its period is $\frac{2\pi }{3}$.
