If you zoom in onto a quarter of the square, say, the top right corner, and by breaking it down into smaller squares, you’ll find that 3/16 of the squares are shaded. Since the entire square consists of 4 such shapes, the ratio stays the same.
Thus, we can see that 3/16 is not shaded, and thus 13/16 is shaded.
Hence, 3/16 = 81cm^2
13/16 = 351cm^2