using cosine rule
[tex]\begin{gathered} s^2=r^2+q^2-2rq\cos S \\ s^2=910^2+370^2-2\times910\times370\cos 31 \\ s^2=828100+136900-336700\times0.8571673007 \\ s^2=965000-288608.230146 \\ s^2=676391.769854 \\ s=\sqrt[]{676391.769854} \\ s=822.430404262 \\ s=822\operatorname{cm} \end{gathered}[/tex]
