Chapter 33 - Wave Optics - Exercises and Problems - Page 956: 35

$d = 0.40~mm$

Note that there are 11 spaces between the 12 bright fringes. Thus we can use $m = 11$ We can find the spacing between the slits: $y_m = \frac{m~\lambda~L}{d}$ $d = \frac{m~\lambda~L}{y_m}$ $d = \frac{(11)~(633\times 10^{-9}~m)(3.0~m)}{52\times 10^{-3}~m}$ $d = 4.0\times 10^{-4}~m$ $d = 0.40~mm$