Answer
$\lambda = 500~nm$
Work Step by Step
Note that $y_4 = 2.0~cm$
We can find the wavelength of the light:
$y_m = \frac{m~\lambda~L}{d}$
$\lambda = \frac{y_m~d}{m~L}$
$\lambda = \frac{(0.020~m)~(0.20\times 10^{-3}~m)}{(4)~(2.0~m)}$
$\lambda = 5.0\times 10^{-7}~m$
$\lambda = 500~nm$