You may have correctly used the definition of secant, but you did not use the Pythagorean Theorem. You can draw a triangle with $\displaystyle{a}={7}\usepackage{cmbright}\Huge\$ and $\displaystyle{o}={8}\usepackage{cmbright}\Huge\$ . Then use the Pythagorean Theorem to find the length h, of the hypotenuse:

$\displaystyle{{7}}^{{2}}+{{8}}^{{2}}={{h}}^{{2}}\usepackage{cmbright}\Huge\$

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