Calculate ΔrG∘ at 298 K for NH3 (g) + HBr (g) →NH4Br (s)

You need information that you haven't provided in the question. Perhaps your text has a table of ΔGof for these compounds. In that case,

ΔGorxn = Sum of ΔGof products - ΔGof reactants

Just plug in the values that you find and calculate the free energy change of the reaction