Can someone explain Optimization in Calculus?

All of what you've done is perfectly correct. The goal of optimization is to take the derivative of a function so you can find out when the slope is zero. This place marks where it is at a maximum or a minimum. So take the derivative of the equation you came up with. A'=40-w. Set A' equal to zero because you're trying to find when the slope is zero because that's where the (local) maximum is at. 0=40-w. So w=40. Which means that l=20. This is the maximum, I'm not sure how to find the minimum but I hope this helped.