rate of change of an area

I am asked to find the rate of change of the area of a rectangle, given it's length is 15 ft, and increasing at 3 ft/sec, and it's width is 6 ft, increasing at 2 ft/sec.

Rate of change of the area (dA) should just be the product of the other two rates (dx*dy) right? or am I missing something? Please help.

The area is given by A=LW

Therefore, by the product rule:

hmmm... I am unfamiliar with this rule. Or at least, I may need a refresher to remember it.