Answer:
The area of a rectangle is the product of its width and length.

Now, in our case, the width is the vertical distance of the rectangle and it is found by counting the boxes from (-8, -4) to (-8,6). There are 10 boxes from the given points; therefore the width of the rectangle is w = 10.
Now the length is the distance between (-8,-4) and (6, -4). Counting the boxes we find that there are 14 boxes; therefore, L = 14.
With the value of width and length in hand, we now calculate the area

Hence, the area of Jamal's deck is 140 square units.