Find the Frequency of the Simple Harmonic Motion Described by the Function

Multiple Choice

Find the frequency of the simple harmonic motion described by the function below. $$d = - 3 \cos ( 18 \pi t )$$

A) 9
B) $- \frac { 2 \pi } { 3 }$
C) $18 \pi$
D) $- \frac { 2 } { 3 }$
E) $- 3$