And we could also verify that it is concave downwards at that point using the second derivative test, by showing that the second derivative is negative there. So let's do that, let's look at the first and second derivatives of our acceleration, of our acceleration function. So, and I'll switch colors, that one's actually a little bit hard to see. So, the first derivative, the rate of change of acceleration is going to be equal to, so this is negative six t plus 12. Now let's think about when does this thing equal zero? Well, if we subtract 12 from both sides, we get negative six t is equal to negative 12. Divide both sides by negative six, you get t is equal to two.