I need help solving this equation. Can anyone help?
Find the least amount of material needed to make a square-based open box that has volume of 4000 cubic meters.
Again, draw a picture. Suppose it is $y$ tall, with base side length $x$. The area of the base is $x^2$, and the sides have area $xy$, so the total amount of material is $x^2+4xy$. The volume is $4000=x^2y$, which you can solve for $y$ to get an expression for total amount of material in terms of $x$. Then it is just like the other problems.