Refer to a $\displaystyle{{30}}^{\circ}-{{60}}^{\circ}-{{90}}^{\circ}\usepackage{cmbright}\Huge\$ triangle and use the definition of cosine with the given triangle:

$\displaystyle\frac{{c}}{{7}}={{\cos{{30}}}}^{\circ}=\frac{\sqrt{{3}}}{{2}}\Rightarrow{2}{c}={7}\sqrt{{3}}\Rightarrow{c}=\frac{{{7}\sqrt{{3}}}}{{2}}\usepackage{cmbright}\Huge\$