Finding electric field amplitude from laser force on reflecting surface

[Wyoming Physics]

Problem we are looking at :A laser beam has diameter 1.30 mm . The beam exerts a force of 3.7×10−9 N on a totally reflecting surface.

What is the amplitude of the electric field of the electromagnetic radiation in this beam? (Emax)
Relevant Formulas...etc.:
I have the formula I=(Emax)^2/(2*c*u) .. where c is the speed of light (3E8) and u is (4piE-7)
I= P/A , where I= intensity

My attempt at the problem:
I am stumped because the problem would be easy if I was given a power although looking at the problem I am not given power, rather, I am given a force . Do I just substitute P(power)= F(force)*C(speed of light)
- Then from there plug (F*C)/A into the I(intensity) for E max.

 

[Cutter Ketch]

Hmmm ... it seems strange they would assign you this problem without first presenting the proper relations. Are you sure you don't have a formula relating force or perhaps pressure (hint hint) to power or intensity, perhaps one that looks very similar to the intensity formula you posted? Ok, just so we're clear, did you ever discuss radiation pressure in class?