A single slit of width b is illuminated by a coherent monochromatic light of wavelength $\mathrm{\lambda }$ . If the second and fourth minima in the diffraction pattern at a distance 1m from the slit are at 3 cm and 6 cm respectively from the central maximum, what is the width of the central maximum (in cm)? (i.e. distance between first minimum on either side of the central maximum)