Use the Formula $a \cos B x + b \sin B x = A \cos ( B x - C )$

Multiple Choice

Use the formula $a \cos B x + b \sin B x = A \cos ( B x - C )$ $\text { where } A = \sqrt { a ^ { 2 } + b ^ { 2 } } \text { and } \tan C = \frac { b } { a } \text { with } - \frac { \pi } { 2 } < C < \frac { \pi } { 2 }$ to determine the period of f (x) . $f ( x ) = 7 \cos 12 x - 7 \sin 12 x$

A) $\frac { \pi } { 7 }$
B) $\frac { \pi } { 6 }$
C) $\frac { \pi } { 4 }$
D) $6 \pi$
E) $\frac { \pi } { 12 }$