I need to know how to work out the area of a quadrilateral with to coordinates of A(-7,0) B(2,3) C(5,0) D(-1,-6).

Would I be right in thinking I would do this:

A and C lies on the x-axis.

B and D are heights

Split as two triangles and sum (ABC +ADC)

=0.5*12*3+0.5*12*6=54

Would this be correct if not where am I going wrong?

Thanks.