Given a cuboid, the diagonal cuts a face of the cuboid into 2 right-angled triangles.
Hence, using the Pythagoras theorem, we have
\(9^{2} = 4^{2} + x^{2}\)
\(81 = 16 + x^{2}\)
\(x^{2} = 81 - 16 = 65\)
\(\therefore x = \sqrt{65} cm\)