The width of a swimming pool is 12 feet less than the pool's length. the area of the pool is 540 square feet. what are the dimensions of the pool?

Let
x----------> the width of the pool
y---------> the length of the pool

we know that
x=y-12-----------> equation 1
540 ft²=x*y ----------> equation 2

substitute 1 in 2

540=[y-12]*y-----> y²-12y-540=0

using a graph tool-----> to resolve the second order equation
see the attached figure

the solution is 
y=30 ft
x=y-12------> x=30-12------> x=18 ft

the answer is
 the width of the pool is 18 ft
 the length of the pool is 30 ft