if normal blood cholestrol level is 5.8 mM

Question

and a treatment caused a drop of 1.0 mM with a standard deviation of the difference of 2.0 mM
what is the minimum number of individuals that should be used in the drug trial to determine whether or not it is effective

i know you have to calculate t

i got t = mean of difference/s(difference)

which gave me t =0.5 but this would give n as infinity

this doesnt seem to be right but im not sure what or if ive gone wrong

thanks