$${F_{vas} = (M + m)g – F_{par} = (2,7\, \mathrm{kg} + 1,8\, \mathrm{kg})(9,8\, \mathrm{m/s^2}) – 15,44\, \mathrm{N} = 28,66\, \mathrm{N} \approx 29\, \mathrm{N}}$$