Use the Formula $a \sin B \theta + b \cos B \theta = \sqrt { a ^ { 2 } + b ^ { 2 } } \cos ( B \theta - C )$

Multiple Choice

Use the formula $a \sin B \theta + b \cos B \theta = \sqrt { a ^ { 2 } + b ^ { 2 } } \cos ( B \theta - C )$ ,where $C = \arctan ( a / b ) , a = 9 , b = 2 , B = 2$ to rewrite the trigonometric expression in the following form.​ $y = \sqrt { a ^ { 2 } + b ^ { 2 } } \cos ( B \theta - C )$ ​ $a \sin B \theta + b \cos B \theta$ ​

A) $\sqrt { 85 }$ $\cos ( 3 \theta - 1.3521 )$
B) 9 $\cos ( 3 \theta + 1.3521 )$
C) 9 $\cos ( 3 \theta - 1.3521 )$
D) $\sqrt { 85 }$ $\cos ( 3 \theta + 1.3521 )$
E) 2 $\cos ( 3 \theta - 1.3521 )$