It might help to turn the image sideways so that it is rotated 90 degrees. Have the triangle facing upward with (x1,y2) and (x1,y1) as the base points. The base has a length of (y2-y1) as this is the difference of y coordinate values. The x coordinate being the same indicates we simply subtract.
The height is (x3-x1) which is the difference of the x coordinates for either left point and the right point
So, area = (1/2)*base*height area = (1/2)*(y2-y1)*(x3-x1) which is why choice D is the answer
