There is a formula for the area of a triangle given the co-ordinates of its vertices; it is given by a 3 x 3 determinant (which I cannot easily write)
x1 y1 1
x2 y2 1
x3 y3 1
Multiply the value of this determinant by 1/2
The expansion of this determinant is (1/2)(x1y2 + x2y3 + x3y1 - x3y2 - x2y1 - x1y3) Good luck!
Otherwise you can easily compute the length of the 3 sides and then use Heron's formula, which is
A = sqrt[s(s-a)(s-b)(s-c)] where a, b and c are the sides and s = (1/2)(a + b + c).