URGENT:

The neon sign in a street emits radiation of wavelength 590 nm. What will be the mass of one photon of this wavelength?

Here we will use the relation  

 λ = h / mc
m = h / $\lambda$c

where  λ is the wavelength of light, h is the planck's constant, m is the mass of photon and c is the speed of light.
h = 6.626 X 10^-34 Js, c = 3 x 10⁸ m/s. Given λ = 590 x 10^-9 m. 
Putting the value in formula:
m = h / $\lambda$c
m = 6.626 X 10^-34 Js / 590 x 10^-9 m x 3 x 10⁸ m/s = 0.0037 x 10^-33 kg = 3.7 x 10^-30 kg