We use the vertical side as the base: it is a vertical segment, so it's length is the difference of the y coordinates of its endpoints:[tex] b = y_2-y_1[/tex]The height would be the segment starting from [tex](x_3,y_3)[/tex], perpendicular to the base. Since this is a horizontal segment, its length is the difference of the x coordinates of its endpoints:[tex]h = x_3-x_1[/tex]So, the area is given by[tex]A = \dfrac{bh}{2} = \dfrac{(y_2-y_1)(x_3-x_1)}{2}[/tex]