In 18 hours there are a total of 13,500 bacteria.
First, you'd find the difference between 6,500 and 5,000 (6,500 - 5,000 = 1,500). Next you'd divide 1,500 by 2 to find how many grow per hour (1,500/2 = 750). Finally, you multiply 750 by 18, to show how many will be present in 18 hours.
I hope this helps!