Roberto has been studying a type of bacteria that doubles every month. originally, there were 3 bacterial cells. he wants to know how many there will be after 43 months. which equation should he use?

Not sure if there should be options listed, but here is an equation he should use:
f(t) = 3*2^t, where t= number of months since first observation.
or
f(t)= 3*2^(t-1), if the moment of observation is t=1

If this isn't an availabe option, plug in a few values for t (number of months) and make sure they match our values
Based on the first equation:
When t = 0 (when Roberto first observes the bacteria):
f(0) = 3*2^0 = 3*1 = 3
f(1)=3*2^1 = 3*2  = 6
f(2)= 3*2^2 = 3*4 = 12
and so on until...
f(43)=3*2^43 = 26,388,279,066,624 That's a lotta bacteria!