For this case we have a function of the form:

Where,
A: initial population of bacteria
b: growth rate
x: number of hours
Since the diameter is double every hour, then:

We must now look for the value of A.
To do this, we evaluate an ordered pair of the graph:
For (1, 2):

Clearing A we have:


Then, the function is given by:

For after 9 hours we have:

Answer:
the diameter of the bacteria after 9 hours is:
