Answer is: 15,21 liters of nitrogen is needed.
Chemical reaction: N₂ + 3H₂ ⇄ 2NH₃.
V(H₂) = 45,0 L.
V(N₂) = ?
n(H₂) = V(H₂) ÷ Vm.
n(H₂) = 45 L ÷ 22,4 L/mol
n(H₂) = 2,01 mol.
from chemical reaction: n(N₂) : n(H₂) = 1 : 3.
n(N₂) = 0,67 mol.
V(N₂) = 0,67 mol · 22,4 L/mol.
V(N₂) = 15,21 L.