Precalculus (6th Edition) Blitzer

Please note that the absolute value for sine and cosine removes the lower half of the graph, which is below the $x$-axis, and flips it above. This results in two complete cycles, where there had been one. So, since the period of $3\cos \frac{2x}{3}$ is $\frac{2\pi }{\frac{2}{3}}=3\pi$, the period of $y=\left |3 \cos \frac{2x}{3} \right |$ will become $\frac{3\pi }{2}$.