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 $\cos s = \frac { \sqrt { 5 } } { 5 }$ , with $\mathrm { s }$ in quadrant $\mathrm { I }$ , and $\sin \mathrm { t } = - \frac { \sqrt { 3 } } { 2 }$ , with $\mathrm { t }$ in quadrant IV.

A) $\frac { \sqrt { 5 } + 2 \sqrt { 15 } } { 10 }$
B) $\frac { \sqrt { 5 } - \sqrt { 15 } } { 9 }$
C) $- \frac { \sqrt { 5 } } { 10 }$
D) $\frac { \sqrt { 5 } - 2 \sqrt { 15 } } { 10 }$