A 2.0 kg particle moves along an x axis, being propelled by a variable force directed along that axis. Its position is given by x = 3.0m + (4.0 m/s)t + ct2 – (2.0 m/s3)t3, with x in meters and t in seconds. The factor c is a constant. At t = 3.0 s, the force on the particle has a magnitude of 36N and is in the negative direction of the axis. What is c?