Chapter 14 Trigonometric Graphs, Identities, and Equations - 14.1 Graph Sine, Cosine, and Tangent Functions - 14.1 Exercises - Skill Practice - Page 913: 14

See below.

The amplitude and period of a graph $y=a\cos (bx)$ or $y=a\sin(bx)$ can be obtained by (if $a,b$ are non-zero): amplitude: $|a|$, period: $\frac{2\pi}{|b|}$. Hence the period in this case is: $\frac{2\pi}{|b|}$, which here is: $3\pi$