Perhaps you referred to a $\displaystyle{{45}}^{\circ}-{{45}}^{\circ}-{{90}}^{\circ}\usepackage{cmbright}\Huge\$ triangle. However, you can use the fact that $\displaystyle{\sin{{M}}}=\frac{{8}}{{16}}=\frac{{1}}{{2}}\usepackage{cmbright}\Huge\$ to conclude that this is a $\displaystyle{{30}}^{\circ}-{{60}}^{\circ}-{{90}}^{\circ}\usepackage{cmbright}\Huge\$ triangle. You can then use the definition of cosine to find the value of p.