Answer
$\lambda = 573~nm$
Work Step by Step
The spacing between these two minima is $\Delta y = \frac{4~\lambda~L}{d}$
We can find the wavelength of the light:
$\Delta y = \frac{4~\lambda~L}{d}$
$\lambda = \frac{\Delta y~d}{4~\lambda}$
$\lambda = \frac{(5.5\times 10^{-3}~m)(0.25\times 10^{-3}~m)}{(4)(0.60~m)}$
$\lambda = 5.73\times 10^{-7}~m$
$\lambda = 573~nm$
