You found the value of $\displaystyle{\sec{{B}}}\usepackage{cmbright}\Huge\$ . First use the Pythagorean Theorem to find b:

$\displaystyle{{b}}^{{2}}+{{6}}^{{2}}={{7}}^{{2}}\usepackage{cmbright}\Huge\$

After solving for b, use the definition of sine:

$\displaystyle\text{sine}=\frac{\text{opposite}}{\text{hypotenuse}}\usepackage{cmbright}\Huge\$