It appears that you just added the two leg lengths. You need to use the Pythagorean Theorem: $\displaystyle{{a}}^{{2}}+{{b}}^{{2}}={{c}}^{{2}}\usepackage{cmbright}\Huge\$ .

Substitute the values $\displaystyle{a}={8}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={15}\usepackage{cmbright}\Huge\$ and then solve for c.