Find the Exact Value of the Expression Using the Provided $\cos ( s - t )$

Find the exact value of the expression using the provided information.
-Find $\cos ( s - t )$ given that $\sin s = - \frac { \sqrt { 3 } } { 3 }$ , with $s$ in quadrant IV, and $\sin t = - \frac { \sqrt { 5 } } { 6 }$ , with $t$ in quadrant
IV.

A) $\frac { \sqrt { 55 } - \sqrt { 15 } } { 18 }$

B) $\frac { \sqrt { 186 } - \sqrt { 15 } } { 18 }$

C) $\frac { \sqrt { 186 } + \sqrt { 15 } } { 18 }$

D) $\frac { \sqrt { 55 } + \sqrt { 15 } } { 18 }$