[tex]D[/tex] is the determinant of the coefficient matrix:
[tex]D=\begin{vmatrix}3&-1\\-3&5\end{vmatrix}=3\times5-(-1)\times(-3)=12[/tex]
[tex]D_x[/tex] is the determinant of the coefficient matrix, but this time you replace the [tex]x[/tex]-coefficients with the values on the right hand side of the equations:
[tex]D_x=\begin{vmatrix}-1&-1\\13&5\end{vmatrix}=(-1)\times5-(-1)\times13=\boxed{8}[/tex]
