"A 50 story building is being planned. It is to be 200 m high with a base of 40 m by 70 m. Its total mass will be about 1.8^7 kg, and its weight therefore 1.8 x 10^8 N. Suppose a 200 km/h wind exerts a force of 950 N/m^2 over the 70 m wide face. Calculate the torque about the potential pivot point, the rear edge of the building (where F_E acts in) and determine whether the building will topple. Assume the total force of the wind acts at the midpoint of the building's face, and that the building is not anchored in bedrock." Apparently the answer is ∑τ = F_A(1000 m) - mg(20 m) = -2.2x10^9 mN but I have no idea how to derive this. Please explain!