Q.Using Cramer's Rule, solve $2x-y=5$ and $x+3y=6$.

The same three-determinant procedure applies to any two-variable system, regardless of which coefficients are negative.

Forming $D$

Since $D\neq0$, a unique solution exists.

Forming $D_x$

Forming $D_y$

Solving

$x=\frac{21}{7}=3 \qquad y=\frac{7}{7}=1$ …

Mishandling the double-negative when expanding a determinant with a $-1$ coefficient, e.g. computing $2(3)-(- …