Find the cross product of $\begin{pmatrix} 5 \\ 2 \\ -6 \end{pmatrix}$ and $\begin{pmatrix} 1 \\ 1 \\ 3 \end{pmatrix}.$
The cross product of $\begin{pmatrix} 5 \\ 2 \\ -6 \end{pmatrix}$ and $\begin{pmatrix} 1 \\ 1 \\ 3 \end{pmatrix}$ is
\[\begin{pmatrix} (2)(3) - (1)(-6) \\ (-6)(1) - (3)(5) \\ (5)(1) - (1)(2) \end{pmatrix} = \boxed{\begin{pmatrix} 12 \\ -21 \\ 3 \end{pmatrix}}.\]