Answer:
Use this OK bro.
Perimeter of rectangle = 2(l+w)
So, 54cm = 2(l+9cm)
=> 54cm = 2l + 18cm
=> 54cm - 18cm = 2l
=> 36cm = 2l
=> 36cm/2 = l
=> 18cm = l
as you already know, to get the inverse of any expression we start off by doing a quick switcheroo on the variables and then solve for "y".