9) B

n(H2) = 9/2 = 4.5 moles
n(N2) = 56/28 = 2 moles

Ratio of H2:NH3 = 3:2
4.5 moles of Hydrogen reacts with 3 moles of Ammonia

We then use the formula:
moles = mass/Mr
moles = 3
mass = unknown
Mr = 14+3 = 17

moles = m/Mr
m = moles x Mr = 3×17 = 51 grams