An array of many identical slits placed close together with uniform spacing is called a grating.
In a particular grating the slits are narrow enough to diffract the light passing through into a broad wavefront. The waves coming from the many slits interfere, producing bright spots on the screen wherever all these waves arrive in phase.
The slide containing the grating in the laser beam is mounted, the set-up is adjusted to produce a bright red diffraction pattern.
You mark the positions of the central maximum and of three diffraction maximums on either side of it. Then you can measure the distances (x1, x2, x3, ....) of the maximums from the central maximum. What would be reasonable estimates for these values?
Estimate the distance L from the grating to the screen; this will allow you to calculate the diffraction angle θ for each maximum (tan theta; n = xn/L).
How would you calculate a value for the wavelength lambda; of the laser light for each of your angles theta; n?