The formula for electric flux is:
electric flux = (charge in) / (permittivity of free space)
where, permittivity of free space = 8.854*10^-12 C^2/N*m^2
and the charge in the cube is = +3.15 nC = 3.15*10^-9 C
plugging these values into the equation we get:
electric flux = (3.15*10^-9 C) / (8.854*10^-12 C^2/N*m^2) = 355.77 N*m^2/C
electric flux = 355.77 N*m^2/C
No, Gauss's law cannot be used to find the electric field on the surface, it is only used to calculate the electric flux through a closed surface.