This can be expressed as exponential growth of the form:
f=ir^t, f=final amount, i=initial amount, r=common ratio (rate), t=time
In this case we are given the point (2, 6500) and i=5000 so we can solve for the rate...
6500=5000r^2 divide both sides by 5000
1.3=r^2 take the square root of both sides and note that we know r>0
r=1.3^(1/2) so our equation becomes:
f=5000(1.3)^(1/2)^t and knowing that (a^b)^c=a^(b*c) we can say:
f=5000(1.3)^(t/2) so for t=18
f=5000(1.3)^9
f≈53022 (to nearest whole bacteria)