Z64 is a cyclic group generated by 1, so its image under any homomorphism must also be cyclic. However, Z4 x Z8 is not cyclic, so there cannot exist a homomorphism from Z64 onto Z4 x Z8. Any homomorphism from Z64 cannot have an image of Z4 x Z8 since the image must be cyclic and Z4 x Z8 is not cyclic.