Chapter 22 - Wave Optics - Exercises and Problems - Page 650: 33

$L = 1.6~m$

Note that $y_4 = 2.0~cm$ We can find the distance $L$ from the slits to the screen: $y_m = \frac{m~\lambda~L}{d}$ $L = \frac{y_m~d}{m~\lambda}$ $L = \frac{(0.020~m)~(0.20\times 10^{-3}~m)}{(4)~(620\times 10^{-9}~m)}$ $L = 1.6~m$