So, we know that:
Every 12 hours, the population doubles.
We start with 1000 bacteria.
We want to find the number of bacteria after 72 hours (3 days).
72 / 12 = 6, so we know the population doubles 6 times.
So set up a formula:
It doubles six times, so we must do 2^6 * 1000 = P
P = 64,000 bacteria, or 6.4 x 10^4 in scientific notation.