You may have correctly used the definition of secant, but you did not use the Pythagorean Theorem. You can draw a triangle with $\displaystyle{o}={3}\usepackage{cmbright}\Huge\$ and $\displaystyle{h}={5}\usepackage{cmbright}\Huge\$ . Then use the Pythagorean Theorem to find the length a, of the adjacent side:

$\displaystyle{{3}}^{{2}}+{{a}}^{{2}}={{5}}^{{2}}\usepackage{cmbright}\Huge\$

Solve this equation for a. Then use the definition of secant.