A) Well if you start with 5, and they triple with every hour then just write out a table of how many there are after 0, 1, 2, 3, 4 and 5 hours. E.g 0 hours - 5, 1 hour - 15.
B) So it's asking you to create a formula to describe the growth of the mould. I'll give you a hint, it involves using n (the time in hours) as a power, since the growth is exponential.
C) If you can create this formula it's easy, just plug in that it equals 1,000,000 and you should be able to find n.
See how you get on.