Check the picture below.
so, the figure is really one big square, a 10x10, with four triangles in it, now, if we just get the area of the square, well 10x10 is just 100 in², and then get the areas of those three triangles, the yellow, purple and orange ones, and subtract their area from that of the larger square, what's leftover is the shaded triangle. By subtracting the areas from the larger area, we're in essence making a big hole in the larger area, and what's leftover is what was never subtracted.
