Sarah decides to put a bird feeder in her back garden. That day she sees $6$6 birds use the feeder. The next day she sees $12$12 birds, and on the third day she sees $18$18 birds.
If the number of birds continues to follow the pattern, how many birds will Sarah see on the fourth day?
If the pattern continues, write an expression for the number of birds Sarah will see after $y$y days.