The following question refers to the following expression for a particle's position:

X = 3t^2 - 4t + 3

What is the particle's acceleration after 3 seconds?

I keep getting 1 m/s2 but that's not an answer choice.
I found instantaneous velocity at t=0 (3 m/s) and at t = 3 (6 m/s) and just calculated change in velocity over change in time....

Where am I going wrong?