Answer
$B=1000\cdot(2)^{t/3}$
$65,536,000$ bacteria
Work Step by Step
We want an exponential model, $B=Ab^{t}$
At the start, $t=0$ hours, and the initial size is $A=1000$
So, $B=1000b^{t}$
Given $(3,2000)$, the $1000$ doubles to $2000$ in $t=3$ hours, so we have
$\left\{\begin{array}{l}
2000=1000b^{3}\\
b^{3}=2000/1000=2\\
b=2^{1/3}
\end{array}\right\}$
Thus
$B=1000\cdot[2^{1/3}]^{t}$
$B=1000\cdot[2]^{t/3}$
In two days ($t=48$ hrs) there will be
$B=1000(2)^{48/3}=65,536,000$ bacteria