This forum doesn't permit giving the full answer, but what you would do is to differentiate you would use the standard 'times by the power, subtract 1 from the power' rule. This gives you dQ/dL. Then plug in L=16 to get your answer. I can't help you much with interpreting the result as that requires subject-specific knowlege.
If you get stuck, post your workings so far so we can help you.