Thanks very much Alan.
I am sure you do know what you are talking about.
We know the original area is 2.5
We know (-1,0) transforms to (-1,sqrt3) and (0,1) transforms to (sqrt3,1)
We know that the distance between the original 2 points is sqrt2 units
We know that the distance between the corresponding transformed two points is sqrt{8}
the ratio of new to old side lengths is sqrt8/sqrt2 = 2
square it and we get the ratio of the areas is 4
The original area is 2.5 so the new area must be 10
NOW we agree :))