Let R be the triangle with vertices at ($x_1$, $y_1$) , ($x_2$, $y_2$), ($x_3$,$y_3$).
Show that {area of triangle} = 1/2 det $ \begin{pmatrix} x_1 & y_1 & 1 \\ x_2 & y_2 & 1 \\ x_3 & y_3 & 1 \end{pmatrix}$
Im not sure how to even start. can someone help me?