Answer
$\Delta[NH_3]/\Delta t=0.0003\ mol/L.s$
Work Step by Step
From stoichiometry:
$-\frac{\Delta[N_2]}{\Delta t}=-\frac{\Delta[H_2]}{3\Delta t}=\frac{\Delta[NH_3]}{2\Delta t}$
$\Delta[NH_3]/\Delta t=2/3\times4.5\times10^{-4}\ mol/L.s=0.0003\ mol/L.s$