# Find force from a function of displacement?

An object with mass m moves along the x-axis. Its position as a function of time is given by x(t)=At-Bt^3, where A and B are constants. Calculate the net force on the object as a function of time.

I have no idea as to how to begin.
Thanks for anyone's help.
I dont know !&@#%^@%!$^$(! about doing this kind of calculus problem.