Answer: [tex]3\leq x \leq 18[/tex]
Step-by-step explanation:
Given, the length of the rectangular dog run is 18 feet.
Formula: Perimeter = 2 ( length + width)
Let width be x.
Then, Perimeter = 2 (18 + x ) feet
The perimeter of the dog run must be at least 42 feet and no more than 72 feet.
That is
[tex]42\leq 2 (x+18)\leq 72[/tex]
Divide inequality by 2, we get
[tex]21\leq 18+x\leq 36[/tex]
Subtract 18 from each side, we get
[tex]3\leq x \leq 18[/tex]
Required inequality : [tex]3\leq x \leq 18[/tex]