Find the Specified Quantity $y = - 3 \cos \left( \frac { 1 } { 2 } x + \frac { \pi } { 3 } \right)$

Multiple Choice

Find the specified quantity.
-Find the period of $y = - 3 \cos \left( \frac { 1 } { 2 } x + \frac { \pi } { 3 } \right)$ .

A) $3 \pi$
B) $\frac { 2 \pi } { 3 }$
C) $4 \pi$
D) $2 \pi$