First find the lengths of each side (square root of 256 is 16, so each side of the bigger square is 16 cm) of each square (square root of 64 is 8, so each side of the smaller square is 8 cm). Then, since each y value is the same and equal, 16 (length of one side of bigger square) minus 8 (length of one side of smaller square), divided by 2 (because there are 2 y values per length), you get 4 cm. Hope that helps!
