Understood Parenthesis, Minimize Cost, Finding when increasing at greatest rate

Hey everyone! I'd greatly appreciate some thoughts on these problems. Thanks in advance. (Rofl)

1) *Inventory Cost.The cost of inventory depends on the ordering and storage costs according to the inventory model:*

$\displaystyle C = \bigg(\frac{Q}{x}\bigg)s + \bigg(\frac{x}{2}\bigg)r$

Determine the order size that will minimize the cost, assuming that sales occur at a constant rate, Q is the number of units sold per year, r is the cost of storing one unit for 1 year, s is the cost of placing an order, and x is the number of units per order.

I have never found the minimum with so many variables. Can someone give me as starting point and I'll follow onward?

2) Modeling Data. The manager of a store recorded the annual sale S (in thousand of dollars) of a product over a period of 7 years, as show in the table, where t is the time in years, with t = 1 corresponding to 1991.

t....(1)......(2)......(3)........(4).....(5)....( 6).....(7)

S...(5.4)..(6.9)....(11.5)...(15.5)..(19)...(22).. .(23.6)

I was supposed to find a cubic function regression function and I did. But then one part says:

Use calculus to find the time when sale were increasing at the greatest.

Does this mean find the absolute maximum?