Air in a 120 km/h wind strikes head on the face of abuilding 45m wide by 75m high and is brought to rest. if air has a mass of 1.3 kg per cubic meter determine the average force of the wind on the building

From conservation of momentum, the ram force can be calculated similarly to rocket thrust:
 F = d(mv)/dt = vdm/dt.
 In other words, the force needed to decelerate the wind equals the force that would be needed to produce it. 
 v = 120/3.6 = 33.33 m/s 
 dm/dt = v*area*density
 dm/dt = (33.33)*((45)*(75))*(1.3) 
 dm/dt =  146235.375 kg/s 
 F = v^2*area*density
 F = (33.33)^2*((45)*(75))*(1.3) =  4874025 N 
 This differs by a factor of 2 from Bernoulli's equation, which relates velocity and pressure difference in reference not to a head-on collision of the fluid with a surface but to a fluid moving tangentially to the surface. Also, a typical mass-based drag equation, like Bernoulli's equation, has a coefficient of 1/2; however, it refers to a body moving through a fluid, where the fluid encountered by the body is not stopped relative to the body (i.e., brought up to its speed) like is the case in this problem.

Note that 120 km/hr = 33.33 m/s 
F = dp/dt 
= (1.3 kg/m^3)(33.33 m/s)(3000 m^2)(33.33 m) So the answer is 4874025 N but the other 4874025 N is one of the three measurements of the fluid volume that contains the mass of air that will be stopped in one second.

Cin carbonh in hydrogeno in oxygen

Independent variable is something that is changed in a experiment.