I don't know how to find this with determinants....but I can tell you how to solve it...
Find the distances between the three points. Label these a, b and c.
Find the semi-perimeter = (s) = (a + b + c) / 2
The area is given by Heron's Formula =
A = √[ s * (s -a) * (s - b) * (s - c) ]
