You found the value of $\displaystyle{\sin{{Q}}}\usepackage{cmbright}\Huge\$ . Remember that the two acute angles in a right triangle will have different trigonometric ratios. First use the Pythagorean Theorem to find p:

$\displaystyle{{7}}^{{2}}+{{p}}^{{2}}={{10}}^{{2}}\usepackage{cmbright}\Huge\$

After solving for p, use the definition of sine:

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