You need to find the distance between the points (10,0) and (15,5) This will give you the width. The length is the distance from (10,0) and (20,-10). The area = width * length.
width = sqrt ( (5-0)^2 + (15-10)^2)
= sqrt50
length = sqrt ((-10-0)^2 + (20-10)^2)
= sqrt 200
So the area of the rectangle = sqrt 200 * sqrt 50
= 10 sqrt2 * 5 sqrt2
= 10*5*2
= 100 square unts Answer