Threshold of sight

sunny.too

1. The problem statement, all variables and given/known data

Hi, I'm having some problems figuring out these questions. I'm hoping someone can please help me out.

Q. The threshold of sensitivity of the human eye is about 100 photons per second. For a wavelength of 557 nm, determine the threshold in watts of power.

2. Relevant equations

E=hf

P= W/t

I don't know which formula to start off with..

3. The attempt at a solution

please help me start it!