(a) Consider that Maggie currently owns 15 albums, this is a constant. Every week, she acquires 6 new albums. You can write this in the following algebraic form:

where t is the number of weeks.
(b) In order to determine the number of albums after 6 weeks, replace t=6 into the previous expression for P(t):

Then, Maggie will have 51 albums.
(c) Replace P(t) = 25 into the expression for P(t) and solve for t,as follow:

Hence, in approximately 1.7 weeks Maggie will have 25 albums.