Answer
The unknown wavelength is $~~465~nm$
Work Step by Step
We can write an expression when there is double slit interference:
$d~sin~\theta_m = m~\lambda$
Let $\lambda_1 = 620~nm$
We can find $\lambda_2$:
$4~\lambda_2 = 3~\lambda_1$
$\lambda_2 = 0.75~\lambda_1$
$\lambda_2 = (0.75)(620~nm)$
$\lambda_2 = 465~nm$
The unknown wavelength is $~~465~nm$
