We know that for each month you're a part of the group, you're paying $25, so we can model that as [tex]25x[/tex], where [tex]x[/tex] is the number of months that you have been a part of the group.
We also know that you have to pay a one-time fee of $100 to join the group, so that should be added to the monthly fees you are paying, which would be [tex]25x + 100[/tex].
Setting this to [tex]y[/tex] gives you the equation for how much money you pay for the group membership:
[tex]y = 25x + 100[/tex]
