Find the Exact Value Under the Given Conditions $\sin \alpha = \frac { 3 } { 5 } , \frac { \pi } { 2 } < \alpha < \pi ; \quad \cos \beta = \frac { 2 } { 5 } , 0 < \beta < \frac { \pi } { 2 }$

Multiple Choice

Find the exact value under the given conditions.
- $\sin \alpha = \frac { 3 } { 5 } , \frac { \pi } { 2 } < \alpha < \pi ; \quad \cos \beta = \frac { 2 } { 5 } , 0 < \beta < \frac { \pi } { 2 }$
Find $\cos ( \alpha - \beta )$

A) $\frac { 8 - 3 \sqrt { 21 } } { 25 }$
B) $\frac { 6 + 4 \sqrt { 21 } } { 25 }$
C) $\frac { 6 - 4 \sqrt { 21 } } { 25 }$
D) $\frac { - 8 + 3 \sqrt { 21 } } { 25 }$