If a preliminary estimate of the population proportion for people who have a given disease and react positively to a new drug is found to be 0.

If a preliminary estimate of the population proportion for people who have a given disease and react positively to a new drug is found to be 0.77, how many medical files should you include in a random sample in order to be 95% sure that the point estimate within a distance of 0.03 from ; that is, the (percent) error is 3%.?

I got 756, but it is the wrong answer. Please show step by step.