The count in a culture of bacteria was 400 after 2 hours and 25,600 after 6 hours.

(a) What is the relative rate of growth of the bacteria population?

(b) What was the initial size of the culture?

My big problem is how to find the rate of growth. Then when I find that I assume I plug in the rate and time=0 to find the initial size of the culture am I correct? I just don't know how to find that blasted rate of growth.