Question 600376
let l be the length of the kitchen and, w its width.

The kitchen is 3 feet longer than its width, means  l = w +3
the area of the floor is 180, means l*w= 180

so we have to solve the following system
 l-w = 3
 l*w=180

At this point , there are many ways to solve the problem.

first , let's do this with the method i consider the easiest one but sometimes the longest one 
  we have to find all pairs of numbers whose product is 180 and look for the one whose difference is 3
we have: ( 1, 180) (2, 90) (3, 60) (4, 45) ( 5, 36), (6, 30) ( 9, 20) ( 10, 18) (12, 15)....
I stopped at ( 12, 15) because  15-12= 3 so the answers are 12ft and 15ft ( we can keep going with the list but that would be a complete waste of time)

Another way 
you pick an equation from the system, write one letter in term of the other one and then plug that in the other equation, and solve the final equation we get.

l= w+ 3 -->  l*w = (w+3)*w = 180
       --> w^2 + 3w -180 = 0
now we use the quadratic formula to solve for w 

w =  (-3 + sqrt(3^2-4(-180)))\2   
  = (-3 + sqrt(9+720))\2
  =  (-3 + 27)/2
  = 24\2
  = 12
 l = w+3 = 12 + 3 + 15
(Notice that i only use the one where the plus inside because the other one with the minus inside, will give us a negative numbers, and we are talking about dimensions so positive numbers. )

 So the dimensions of that kitchen is 12ft and 15ft