how to find area of a triangle with three vertices code example

Example: how to find vertices of triangle if area is given

(1/2)*abs( x1 * (y2 - y3) + x2 * (y3 - y1) + x3 * (y1-y2) ) , where (x1, y1) , (x2, y2) and (x3, y3) are the vertices of the triangle