Area = $\frac{1}{2}$|x1(y2 - y3) + x2(y3 - y1) + x3(y1 - y2)|. This equals zero when the three points are collinear. For the area of a quadrilateral with vertices in order, split into two triangles. The signed version (without absolute value) gives positive area when vertices are counter-clockwise and negative when clockwise.