I guess I could say something also. Personally I think the cross product method is the best way. But if you want another one...

There is something called "Heron''s Formula" for calculating the area of a triangle (any arbitrary triangle). If you know the lengths of all the sides of the triangle (a, b, and c) you can do something like this:

A = sqrt [s*(s-a)*(s-b)*(s-c)]

Where "s" equals the semi-perimeter:

s = (a+b+c)/2

Good luck.