When the light goes into the water, the frequency does not change. So, the wavelength must change. Once you get the new wavelength, you can actually solve the problem by using interference. In this case you are solving for x. This is dispersion.
Dispersion is the separation of colors due to differences in the index of refraction.
Since you are dealing with a refractive phenomena, you will have to use Snell's Law. Find each angle separately. Find the angle using inverse sine. The other leg will be the distance 1.50 meters going from the slit directly to the center of the pattern on the screen. Draw the full triangle.