Answer
$\Delta \lambda=59.5pm$
Work Step by Step
We know that:
$N=1.8\times 1400=2520slits$
We also know that
$\Delta \lambda=\frac{\lambda}{mN}$
We plug in the known values to obtain:
$\Delta \lambda=\frac{450\times 10^{-9}}{3\times 2520}=59.5pm$
