Remember, we're moving along the x-axis, so if our velocity is negative, that means that x is decreasing or we are moving to the left. What is the particle's acceleration, a of t, at t equals three? So pause this video again and see if you can do that. Well, here, the realization is that acceleration is a function of time. It's just the derivative of velocity, which is the second derivative of our position, which is just going to be equal to the derivative of this right over here. And so I'm just going to get derivative of three t squared with respect to t is six t, derivative of negative eight t with respect to t is minus eight and derivative of constants is zero. So it's just going to be six t minus eight.