The central maximum extends to the first minimum on either side.
So the first minimum occurs at 3cm / 2 = 1.5cm = 0.015m to the side from the center of the central maximum.
Now using the formula,
λ = a(y/L)
Where a is the width of the slit, y = distance of first minimum from center, L = distance of screen from slit, and λ = wavelength of the light that strikes a single slit at normal incidence which is unknown here.
a = L x λ/y
a = 1.80 m x (λ / 0.015 m)
a = 120 λ
Use the value of λ for any light and you get the width of slit.