Answer
5.91 mm.
Work Step by Step
Use equation 36.3 because the angle is small.
$$y_1=\frac{x \lambda}{a}$$
The distance between the two dark fringes on either side of the central maximum is $2y_1$.
$$2y_1=2 \frac{x \lambda}{a}=2 \frac{(3.50m) (633\times10^{-9}m)}{0.750\times10^{-3}m}=5.91\times10^{-3}m $$
