Chapter 5 - Trigonometric Functions - Section 5.4 Graphs of the Sine and Cosine Functions* - 5.4 Assess Your Understanding - Page 431: 16

Amplitude= $3$ Period= $\dfrac{2\pi}{3}$

Recall: For $y=A\cos{(ωx)}$, we have: $\text{Amplitude}=|A|\quad \quad \text{and} \quad \quad \text{Period}=T=\frac{2\pi}{ω}$ In $y=-3\cos (3x)$ we have $A=-3$ and $\omega=3$, so Amplitude =$|A|=|-3|=3$ Period $T=\dfrac{2\pi}{\omega}=\dfrac{2\pi}{3}$