I need some help on grade 12 math?

Okay, so I know the answer because it's in the back of my textbook, but I keep getting a different answer. I just need help on how to do it.

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

a) Find the doubling period (I did this and got the right answer: 1.5 hours)

b) Determine the number of bacteria after 8 hours

c) Determine the number of bacteria after 1 hour

The equation used to figure these questions out is: M=c(2)^t/D where M is the total amount, c is the initial amount, 2 is the growth factor, t is the time, and D is the doubling period.

So, I plugged in the known amounts to the equation: M=3000(2)^8/1.5 I think this is right, but I keep getting the wrong answer. The answer is 120 952 by the way. Does anyone know how to do these questions? Did you get the right answer, and if so how did you do it?

Thanks!