We propose a neutron scattering study of the bilayer 2D Mott insulator Sr3Ir2O7. This study is required to resolve both the magnetic structure and interactions and the striking difference of the magnetism of Sr3Ir2O7 in comparison to the monolayer Sr2IrO4. A recent neutron diffraction study has identified high temperature Bragg reflections that they associate with a newly observed high temperature magnetic phase, without definite proof of the magnetic nature of these reflections, or whether this structure causes canting of the low temperature magnetic order. Our Iridium L-edge resonant inelastic x-ray scattering (RIXS) data have indicated a large spin gap with apparent absence of an acoustic mode. Similar data on the same compound has been interpreted in terms of a spin-Hamiltonian very different from its monolayer version in Sr2IrO4, introducing a very large magnetic anisotropy. An alternative explanation is that the acoustic Sr3Ir2O7 mode has no RIXS matrix element at the Iridium L-edge. Our neutron scattering proposal is designed to resolve the magnetic nature of Sr3Ir2O7.