Find the Exact Value of the Indicated Trigonometric Function of θ

Multiple Choice

Find the exact value of the indicated trigonometric function of θ.
- $\tan \theta = - \frac { 21 } { 20 } , \quad 270 ^ { \circ } < \theta < 360 ^ { \circ } \quad$ Find $\cos \theta$

A) $- \frac { 15 } { 17 }$
B) $\frac { - 21 \sqrt { 41 } } { 41 }$
C) $- 15$
D) $\frac { 20 \sqrt { 41 } } { 41 }$