Anotebook has a perimeter of 36 inches and an area of 80 square inches. what are the dimensions of the notebook?

Let L be length and W be width
The perimeter of a rectangle is 2W+2L=36
Thus 2W=36-2L, W=18-L
Area of a rectangle is WL=80
Thus (18-L)(L)=80
18L-L^2=80
L^2-18L+80=0
(L-8)(L-10)
Thus L can either be 8 or 10.
Because WL=80
If L is 8, then W=10
If L is 10, then W=8
So either way, the notebook is 8 by 10

What the heck don’t understand