We would make use of the cosine rule,
[tex]\begin{gathered} C^2=A^2+B^2-2AB\cos C \\ C^2=15^2+10^2-2\times10\times15\times\cos 78 \end{gathered}[/tex][tex]\begin{gathered} C^2=325-62.374\text{ =262.626} \\ C=\text{ 16.206 }\approx\text{ 16.2 in} \end{gathered}[/tex]
