finding sample size with confidence intervals? (homework help)

the question is:
if you were to collect a sampleto form a 95% confidence interval for the population mean, and wished to have a margin of error of 1.5, what sample size would you have to collect? assume that you do not know the population standard deviation.
ive looked all over the chapter in the book and i cant find any formula that would apply for z, t or chi square distributions to find a sample size. any help would be greatly appreciated.:wave: