Chapter 4 - Section 4.6 - Graphs of Other Trigonometric Functions - Exercise Set - Page 610: 34

Please see below.

Please note that since $\sec (Bx-C)=\frac{1}{\cos (Bx-C)}$, the period of the secant function is the same as the period of the cosine function, that is, $\frac{2\pi }{B}$. So the period of the function $y=3\sec x$ is $\frac{2\pi }{1 }=2\pi$.