Find the Exact Value of the Expression Using the Provided $\cos ( \mathrm { s } + \mathrm { t } )$

Find the exact value of the expression using the provided information.
-Find $\cos ( \mathrm { s } + \mathrm { t } )$ given that $\cos \mathrm { s } = - \frac { 1 } { 2 }$ , with s in quadrant III, and $\cos t = - \frac { 3 } { 5 }$ , with $\mathrm { t }$ in quadrant III.

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