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

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

A) $\frac { 3 \sqrt { 3 } - 4 } { 10 }$

B) $\frac { 3 - 4 \sqrt { 3 } } { 10 }$

C) $\frac { 3 + 4 \sqrt { 3 } } { 10 }$

D) $\frac { 3 \sqrt { 3 } + 4 } { 10 }$