Green light can have a wavelength of 543 nm. the energy of a photon of this light is

The energy per photon can be calculated through this formula:
E = hc/λ
where
h is Planck's constant = 6.626x10⁻³⁴ J·s 
c is the speed of light = 3x10⁸ m/s 
λ is the wavelength

The wavelength is given to be 540 nm or also equivalent to 543x10⁻⁹ m.
Substituting the values,

E = (6.626x10⁻³⁴ J·s )(3x10⁸ m/s )/(543x10⁻⁹ m)
E = 3.66x10⁻¹⁹ J/photon