It costs $4 to go to the swimming pool. a membership at the pool costs $21. members pay $3 to use the pool each time. write an algebraic expression for the cost for a member to swim, including the cost of a membership.

The cost of going swimming with a membership can be expressed as C(cost)=3n(number of trips)+21
The cost of going swimming without a membership : C(cost)=4n(number of trips)

Using t for times at the pool and c for total cost. You can use the equation 21+3t=c because they're paying 21 for the membership and 3 additional dollars every time they swim. The cost varies with the number of times in the pool