Find an Equation for the Graph $y = 2 \cos 3 x$

Multiple Choice

Find an equation for the graph.
-

A) $y = 2 \cos 3 x$
B) $y = 2 \cos \frac { 1 } { 2 } x$
C) $y = 3 \cos \frac { 1 } { 3 } x$
D) $y = 3 \cos 2 x$