Answer
$\lambda = 600~nm$
Work Step by Step
Since the distance between the first and tenth minima is 18.0 mm, the distance from the central maximum to the tenth minimum is 19.0 mm.
We can find the angle $\theta$ of the tenth minimum:
$\theta = tan^{-1}~(\frac{1.90~cm}{50.0~cm}) = 0.03798~rad$
We can find the wavelength:
$\theta = \frac{(m+0.5)(\lambda)}{d}$
$\lambda = \frac{d~\theta}{m+0.5}$
$\lambda = \frac{(0.15\times 10^{-3}~m)(0.03798~rad)}{9+0.5}$
$\lambda = 600~nm$
