The human eye can respond to as little as 10^−18 j of light energy. for a wavelength at the peak of visual sensitivity, 550 nm, how many photons lead to an observable flash?

The key equation is going to come from Mr Planck: E=h \nu

Where h is Plancks constant; and ν is the frequency. This equation gives you the energy per photon at a given frequency. Alas, you're given wavelength, but that's easy enough to convert to frequency given the following equation:

c= lambda / nu

where c is the speed of light; λ (lambda) is the wavelength; and ν is again frequency. As soon as you know the energy of a photon with a wavelength of 550nm, you should know how many photons you would require to accumulate 10^-18J. Be careful with your units.