##6.5/5 = 1.3 MJ kg^-1##
The “Latent heat of vaporization” for a substance(in this case ##NH_3##) is the heat that is required to cause a change in phase of a given mass(not specified)
We are told that ##6.5 MJ## caused this change at the boiling point. Which tells that ALL the heat gained = the heat needed to change in phase.
Hence, ##6.5 MJ## IS the latent heat for vaporization of ##NH_3##
But the question asks for “specific” latent heat,
so we divide by the mass,
## (6.5MJ)/(5kg) = 1.3 MJ kg^-1##
