You don't need to pay attention to the area in this case. We are trying to figure out the mass. What do we need? F= m*a (Force= mass * acceleration of gravity). acceleration of gravity on earth: 9.81m/s^2. so 15000= m*9.81. We need m (Mass) so m= 15000N/ 9.81m/s^2 = 1529.051988 ≈ 1529.05kg.
the mass of the empty plane is therefore 1529.05kg
