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
