TeX source:
\begin{array}{rclcl} u_1 = z^4 &=& 81 &\vert& \pm\sqrt[4]{} \cr z_1 &=& \sqrt[4]{81} = 3 \cr z_2 &=& -\sqrt[4]{81} =-3 \cr\cr u_2 = z^4 &=& 16 &\vert& \pm\sqrt[4]{} \cr z_3 &=& \sqrt[4]{16} = 2 \cr z_4 &=& -\sqrt[4]{16} = -2\end{array}