You may have correctly used the identity $\displaystyle{\sin{{\left(\theta+{2}\pi\right)}}}={\sin{\theta}}\usepackage{cmbright}\Huge\$ to simplify $\displaystyle{\sin{{\left(\frac{{{11}\pi}}{{3}}\right)}}}\usepackage{cmbright}\Huge\$ to $\displaystyle{\sin{{\left(\frac{{{5}\pi}}{{3}}\right)}}}\usepackage{cmbright}\Huge\$ , but forgot what the sign of sine is in the fourth quadrant.