I am so confused on how to set this up then solve please help.

For a rectangle with area 100 to have the smallest perimeter what divisions should it have?

Would I use 2L+2W=P, A=xy, or what. I know it is probably something really simple and I am just not seeing it.

Yes, your formulas are right.

What are the multiples of 100?

10 * 10
5 * 20

Which of those two dimensions will give you the smallest perimeter?