Answer:
x=34
Step-by-step explanation:
90-22=68
68÷2=34
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
Usually one will differentiate the function to find the minimum/maximum point, but in this case differentiating yields:

which contains multiple solution if one tries to solve for x when the differentiated form is 0.
I would, though, venture a guess that the minimum value would be (approaching) 5, since the function would be undefined in the vicinity.
If, however, the function is

Then differentiating and equating to 0 yields:

which gives:

or

We reject x=5 as it is when it ix the maximum and thus,

, for