Answer
$2.1*e^{0.001t}$
Work Step by Step
We can convert the general function of:
$f(t)=A*b^{t}$ as: $f(t)=Q_0*(e^k)^t=Q_0*e^{kt}$
In the exercise, we have:
$f(t)=2.1*1.001^{t}$
In order to get the desired form of the function we have to convert:
$1.001=e^k$
We can use the logarithm for this:
$\log_e{1.001}=k$
$\ln{1.001}=0.001=k$
Therefore the result will be:
$2.1*e^{0.001t}$