Answer:
D = 10.31m
Explanation:
To find the screen distance you use the following formula:
[tex]\Delta y=\frac{\lambda D}{d}\\\\D=\frac{d\Delta y}{\lambda}[/tex] (1)
where delta y is the distance between fringes. You know the screen must have 21 fringes on each side of its center (plus the central one), that is:
[tex]n_{fringes}=2*21+1=43\\\\\Delta y=\frac{4.75m}{43}=0.11m\\\\[/tex]
finally, by replacing the values of the parameters in (1) you obtain:
[tex]D=\frac{(65*10^{-6}m)(0.11m)}{693*10^{-9}m}=10.31m[/tex]
hence, the distance to the screen is 10.31m