Well, if they gave us units, if they told us that x was in meters and that t was in seconds, well, then x would be, well, I already said, would be in meters, and velocity would be negative one meters per second. You might also be saying, well, what does the negative mean? Well, that means that we are moving to the left. 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.