You can put this solution on YOUR website!
the x dimension is the difference in the first two points (where y is constant)
the y dimension is the difference in the 1st and 3rd points (where x is constant)
the area is just x times y __ 7.8*10.2
this works because the rectangle is oriented in line with the axes
__ if it were tilted, you would have to use the distance formula to find the lengths of the sides