Answer
2.4mm
Work Step by Step
Because it is submerged in water, the wavelength will be $\lambda_w=\frac{\lambda}{n}=\frac{470nm}{1.33}=353nm$
$d\sin(\theta)=m\lambda$
For small angles, $\sin(\theta)=\tan(\theta)=\frac{x}{l}$
$x=\frac{m\lambda l}{d}=\frac{1(353.38\times10^{-9}m)(0.400m)}{6.00\times10^{-5}m}=2.4mm$
