Use Identities to Find the Exact Value $\cos ( s - t )$

Use Identities to find the exact value.
-Find $\cos ( s - t )$ given that $\cos s = - \frac { 1 } { 2 }$ , with $s$ in quadrant III, and $\cos t = - \frac { 3 } { 5 }$ , with $t$ in quadrant III.

A) $\frac { - 3 + 4 \sqrt { 3 } } { 10 }$
B) $\frac { 3 + 4 \sqrt { 3 } } { 10 }$
C) $\frac { 3 - 4 \sqrt { 5 } } { 10 }$
D) $\frac { 3 - 4 \sqrt { 3 } } { 10 }$