A plane light wave with wavelength = 0.65m falls normally on a large glass plate whose opposite

Question:

A plane light wave with wavelength λ = 0.65μm falls normally on a large glass plate whose opposite side has a long rectangular recess 0.60 mm wide. Using Fig. 5.19, find the depth h of the recess at which the diffraction pattern on the screen 77 cm away from the plate has the maximum illuminance at its centre.