First chose any 2 random points from the graph, im gonna chose (0,1) and (-4,4). It doesn't matter which one is the x1 or x2, what matter is they have to be in the same order, you can't have (-4,4) as a (x1,y2), That's wrong.
I think that it would be 8 inches because to find the area of a triangle is base times height divided by two. So I just doubled the area and divided by 6.