One way to set up a correct equation is to use the definition of sine:

$\displaystyle{{\sin{{39}}}}^{\circ}=\frac{{y}}{{10}}\usepackage{cmbright}\Huge\$

Solve this equation for y:

$\displaystyle{y}={10}\cdot{{\sin{{39}}}}^{\circ}\approx{10}\cdot{\left({0.6293}\right)}={6.293}\usepackage{cmbright}\Huge\$

This rounds to 6.29.