I had a look at the text, it continues on from Question 4 which has the key piece of information about the electron beam that's directed onto the 'reflecting surface'.

So the electrons in the beam have the same energy as a photon of yellow light -->

a) need to figure out the momentum first before we can do c)

c) determine the number of electrons that are incident on the surface each second in order to produce a pressure of 0.553N m^-2

So Pressure is the force per sq. metre.

The area is

So the force acting on all the electrons to achieve that pressure is:

We also know Newton's second law:

If we consider the force that would act for 1 second, and the momentum that would act for 1 second, and assume that each electron feels the same amount of force, pressure etc, then we can say:

(where k is just a number representing the number of electrons)

electrons.

I think that's the correct reasoning.

edit: hmm, was that what you did as well? I just took a look at what the book had and they had

.

edit: fixed a typo where I had an extra 'k' in the equation when it shouldn't have been there.