Explanation:
the area of a triangle :
baseline × height / 2
it does not matter, if the height is inside or outside (like in this case) of the triangle.
the baseline and height pair is always determined by the right angle between them. if there is no right angle, it is not the height to that side (or the side to that height).
so,
the area is
3 × 2/3 / 2 = 1 ft²