1,\,5)\] and \[(3,\,-7)\] is 21 sq. units \ \[\FRAC{1}{2}\left| \,\begin{MATRIX} x & y & 1 \\ 1 & 5 & 1 \\ 3 & -7 & 1 \\ \end{matrix} \right|=21\] SOLVING; locus of point \[(x,\,y)\] is \[6x+y-32=0\].