I have totally forgotten how to do these questions, so I would really like to check my work...

A bacteria culture starts with 3000 bacteria and grows to a population of 12000 after 3 hours.

a) Find the doubling period.

b) Determine the number of bacteria after 8 hours.

what i did was...

a) 12000 = 3000r^3

and got r=1.59

b) A=3000(1.59)^8

=122545.7

Is that right?