Answer:
a)

b) 50 is the initial population of protists, that is, the number of protists at the beginning of the experiment.
3 is the growth rate which is the factor by which the population of protists grows every hour.
Step-by-step explanation:
An exponential function is generally given as;

where a = initial or starting amount
b = the growth rate which is the quantity by which the initial amount is
multiplied each time t is increased by 1
t = time
a) Given an initial population of bacteria as 100 and we're told that every hour, each bacteria splits into two, we can deduce that;
a = initial amount
b = growth rate = 2
We can go ahead and write the required expression as;

b) Given the below;

If we compare the given function with the exponential function stated above, we can see that;
a = initial amount 50
This means that 50 is the initial population of protists, that is, the number of protists at the beginning of the experiment.
b = growth rate = 3
This means that 3 is the growth rate which is the factor by which the population grows every hour.