Answer:
Φ = B * A
A = .30 m * .12 m = .036 m^2 area perpendicular to direction of flux
Φ = .024 W/m^2 * .036 m^2 = 8.64 * E-4 Webers
Only the bottom of the box is perpendicular to the magnetic field
Φ is the flux into the bottom of the box (and out of)
There is no flux thru any other portions of the box