Let
x--------> the number of sessions
y-------> the total payment
we know that
without membership:
[tex]y=6x[/tex] -------> equation A
with membership:
[tex]y=100+4x[/tex] -------> equation B
using a graph tool------> resolve the system of equations
the solution is the intersection both graphs
see the attached figure
the solution is the point [tex](50,300)[/tex]
that means
For [tex]x=50\ sessions[/tex]
the cost is [tex]\$300[/tex] in both forms
so
to justify buying the membership
[tex]x> 50\ sessions[/tex]
therefore
the answer is
The equation to determine how many sessions you must use the pool to justify buying the membership is
[tex]x> 50\ sessions[/tex]