We know that it cost $9 to go to the pottery place. This is independent of the number of bowls painted.
Then, we know that she have a fixed cost of $9 per day.
We know that each bowl she paints will cost her $3.
Then, we know that she has a variable cost of $3 per bowl.
If she is expected to paint "b" bowls per day, she will have a cost-per-day that is the addition of the fixed $9 and the cost-per-bowl ($3 * b).
Then, she has a (9+3b) dollars per day.
If she is planning to attend 5 days, so the cost is:
C=5 * Cost-per-day = 5*(9+3b) = 5(3b+9)
The total cost for Natalie is 5(3b+9)