We know that the perimeter is the all the sides added up together. Generally, you have

, where l is length and w is width.
For a square, all sides are of the same length, so you can write

, where x is any side.
If the smallest perimeter is 14, then

, so x is 3.5. If the largest perimeter is 72, then

, so x is 18.
Therefore, you have 3.5 ≤ x ≤ 18.
Step-by-step explanation:
f(x) = y= -x²-x+6
When y is max, f'(x) = 0
f'(x) = -2x-1
Equating to 0,
-2x-1= 0
x= -1/2
Put the value of x in f(x)
y = -(-1/2)² -(-1/2) +6
= -1/4+1/2+6
= 6+1/2
y= 13/2