Question

The Poynting vector is given by S(x,t)=a⋅sin2(bx−ct)
what are a ,b ,c ?

Answer 1

a = cu
u = εoEo^2
b = k = wavenumber
c = ω

Answer 2

Careful, in this equation:
a = cu
c is actually the speed of light \(c\) in a vacuum, not the third unknown c of the problem.
so a = \(c\) u

On the other hand, there is no change in this equation:
c = ω