The missing side is 8.
To get the area you need to get the area of the two shapes that are there, they are a rectangle and square
The area of the square is side x side, which is 16 x 16 = 256m2
And the area of the rectangle is side x height, which is 32 x 8 = 256m2
Finally, you add both areas, which is 256 + 256 = 512m2
Or 256 x 2 = 512m2