The number of bacteria in species A at time t is given by: 
.
The number of bacteria in species B at time t is given by
You are asked to find N(t) which is the difference in the species at time t. Difference refers to the answer is a subtraction problem. Thus, we are asked to find N(t) = A(t)-B(t)
We subtract and obtain:
That is,
which is the last answer choice.
With respect to notation, it is common to denote an exponent using ^. So if we want to write
we can write "(.25t)^3"
The number of bacteria in species B at time t is given by
You are asked to find N(t) which is the difference in the species at time t. Difference refers to the answer is a subtraction problem. Thus, we are asked to find N(t) = A(t)-B(t)
We subtract and obtain:
That is,
With respect to notation, it is common to denote an exponent using ^. So if we want to write