hi, ive posted here before with regards to these subjects and im still struggling to grasp the concept. in my book theres an example,

an object moves along a straight line so that its position at time t in seconds is given by x=2t^3-6t (in meters) (t is greater than or equal to 0)

i)find the expresions for velocity and acceleration of the object at time t.

position - x=2t^3-6t

velocity - v=dx/dt=6t^2-6

acceleraration - a=dv/dt=12t

now where does the -6 come from in the velocity part :S and where does is go when you move onto the acceleration? thanks in advcnace...