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}}\Rightarrow{9}+{{a}}^{{2}}={25}\Rightarrow{{a}}^{{2}}={16}\Rightarrow{a}={4}\usepackage{cmbright}\Huge\$

Now use the definition of secant:

$\displaystyle{\sec{{X}}}=\frac{{h}}{{a}}=\frac{{5}}{{4}}\usepackage{cmbright}\Huge\$