Start with trying to figure out rate (r):
P = i•e^rt
5200 = 4000•e^(r•7)
5200/4000 = 1.3 = e^7r
ln 1.3 = ln (e^7r)
.262 = 7r•1, because the ln (e) = 1
r = .262/7 = .0375
Now that you have the rate, plug in that for r and 9 hours as the new t, with again our 4200 as the i
P = i•e^rt = 4200•e^(.0375•9)
P = 4200•1.401 = 5,886 bacteria
