The formula for calculating the diagonal of a cuboid is given by : \(\sqrt{l^2 + w^2 + h^2}\) where w = width, l = length and h = height 9 = \(\sqrt{l^2 + w^2 + h^2}\) 81 = 4\(^2\) + 4\(^2\) + h\(^2\) 81 - 16 - 16 = h\(^2\) h\(^2\) = 49 h = \(\sqrt{49}\) = 7cm NOTE:[l = w = 4 (square base)]