Light of wavelength is diffracted through a single slit of width a, and the resulting pattern

Light of wavelength λ is diffracted through a single slit of width a, and the resulting pattern is viewed on a screen a long distance L away from the slit.

(a) Show that the width of the central maximum on the screen is approximately 2Lλ/a.

(b) If a slit of width 2Lλ/a is cut in the screen and is illuminated, show that the width of its central diffraction maximum at the same distance L is a to the same approximation.