Originally Posted by

**vesperka** Thanks for your help so far, very easy to follow :)

However, I'm stuck on the last step (it's my first day back from winter vacation heh).

To find a local minimum, normally I take the derivative, find the critical points, then create a sign chart to see a sign change from negative to positive.

Now, I'm not familiar with derivatives involving y. I think it has something to do with implicit differentiation which we have yet to actually implement. Ignoring this though, I would find the derivative to be:

S' = -558y^-2 + 3

Instead of using the quotient rule, I rewrote 558/y as 558y^-1. If I'm correct up to here (which I'm guessing I'm not because I'm not sure how to find the derivative with respect to y) what do I do next?