A horizontal force F is applied to a rectangular package having a mass m = 7.5 kg. The magnitude of the force F increases linearly with the distance x, having an initial value equal to the minimum force required to move the package. The package moves over a horizontal surface. When the package has moved a distance of 6 m its velocity is 4 m/sec. If the friction coefficient between the package and the surface is 0.2, determine the maximum value of the force F.