You found the value of $\displaystyle{\tan{\theta}}\usepackage{cmbright}\Huge\$ . First use the Pythagorean Theorem to find h:

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

After solving for h, use the definition of cosine:

$\displaystyle{\cos{\theta}}=\frac{\text{adjacent}}{\text{hypotenuse}}\usepackage{cmbright}\Huge\$