Let's just try it out. When x is just a little bit less than positive 3, what's the sign of f prime prime of x? It's going to be 4 times 9. It's going to be 4 times a positive number. It might be like 2.999, but this is still going to be positive. This is going to be positive when x is approaching 3. Then this is going to be, well, if x is 3, this is 0.