Three identical very dense masses of 4800 kg each are placed on the x axis. One mass is at x1 = -140 cm, one is at the origin, and one is at x2 = 300 cm. What is the magnitude of the net gravitational force Fgrav on the mass at the origin due to the other two masses? Take the gravitational constant to be G = 6.6710^-11 N m^2 /kg^2