We can split the polygon up into two rectangles, one 8x2, and one 1x4 by creating a horizontal line.
We can then find the areas of 16 un^2 and 4 un^2.
I think you can add them up, and get your final answer.
You are very welcome!
If we "filled" in the "missing" part of the rectangle......we would have an area of [ 8 * 3 ] = 24
However.....we have taken a smaller rectangle with an area of (4 * 1) = 4 away from this
So....the total area = 24 - 4 = 20 (square units )