Simple Physics Problem?

1. The problem statement, all variables and given/known data

A 1.2 kg particle moves along an x axis, being propelled by a variable force directed along that axis. Its position is given by x = 3.0 m + (4.0 m/s)t + ct2 - (2.2 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 36 N and is in the negative direction of the axis. What is c?

2. Relevant equations



3. The attempt at a solution


I double dif. to get a = 2c-13.2t and plug in t = 3 => 2c-39.6 right? F=MA so its 36 = (1.2)(2c-39.6)?