In 18 hour it grew 13,500, because it grew 1,500 in 2 hour and if you divide 18 by 2 it is 9 so 1,500 time 2 is 13,500. hopefully I am correct and this helps you.
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.