You may have correctly determined that the reference angle is $\displaystyle{{30}}^{\circ}\usepackage{cmbright}\Huge\$ and then confused tangent with cotangent or made a mistake while simplifying. To find the correct value of $\displaystyle{{\tan{{210}}}}^{\circ}\usepackage{cmbright}\Huge\$ you must find the value of $\displaystyle{{\tan{{30}}}}^{\circ}\usepackage{cmbright}\Huge\$ and then determine the correct sign. To determine the sign, use the signs of x and y in the third quadrant, remembering that: $\displaystyle{\tan{\theta}}=\frac{{y}}{{x}}\usepackage{cmbright}\Huge\$