Using the product role, we get F prime of X equaling the derivative of the first which is one times the second plus the derivative of the 2nd. So we bring down the three. We take the 16 -, X to the one less power and then we have to do the chain role, so the derivative of the inside -1 times the first term of the X. So we have 16 -, X squared in common between those two terms. So we're going to factor out a 16 -, X From there. We get left a 16 - X - 3 X or 16 - 4 X. If I pull out a four from that last parenthesis, I get four times the quantity 16 - X ^2 times the quantity 4 - X. If I think about putting that on a number line, we have our four and we have our 16 from our two parentheses. If I put a number pass 16 to the right, 17/18/19 a million, I'm going to get a negative value because that 16 -, X ^2 will turn positive, but 4 minus something big is going to be a negative at 16. That's going to keep being a negative on the other side because of the quantity squared. That multiplicity at 4:00, it's going to change from a negative to a positive because it's multiplicity is 1. So we know that from negative Infinity to four, the original function will be increasing, from 4 to 16 the original functions decreasing, and from 16 to Infinity the original functions decreasing. So critical points here are going to be 4 and 16.