Answer
$d = 0.200~mm$
Work Step by Step
We can find the width of the slit:
$\Delta y = \frac{\lambda~L}{d}$
$d = \frac{\lambda~L}{\Delta y}$
$d = \frac{(633\times 10^{-9}~m)(1.50~m)}{4.75\times 10^{-3}~m}$
$d = 2.00\times 10^{-4}~m$
$d = 0.200~mm$
