Answer:
a) We need a sample size of at least 3109.
b) We need a sample size of at least 4145.
Step-by-step explanation:
In a sample with a number n of people surveyed with a probability of a success of  , and a confidence level of
, and a confidence level of  , we have the following confidence interval of proportions.
, we have the following confidence interval of proportions.

In which
z is the zscore that has a pvalue of  .
.
The margin of error is:

99% confidence level
So  , z is the value of Z that has a pvalue of
, z is the value of Z that has a pvalue of  , so
, so  .
.
(a) he uses a previous estimate of 25%?
we need a sample of size at least n.
n is found when  . So
. So






We need a sample size of at least 3109.
(b) he does not use any prior estimates?
When we do not use any prior estimate, we use 
So






Rounding up
We need a sample size of at least 4145.