As a matter of fact, there is a neat formula for this problem.
If you understand determinants, you'll like this . . .What is the area of the triangle whose vertices are (0, 0), (14, 6), and (12, 9)?
The area of the triangle with vertices: . is:
. . . . .
For your three vertices:
. . . .
Note: The "determinant bars" also represent absolute value.
. . . . .The determinent can turn out to be negative.
Edit: correct a silly oversight . . .