Answer:
a) 
And rounded up we have that n=2401
b) 
And rounded up we have that n=626
And we see that if we have previous info about p the sample size varies a lot.
Step-by-step explanation:
Previous concepts
A confidence interval is "a range of values that’s likely to include a population value with a certain degree of confidence. It is often expressed a % whereby a population means lies between an upper and lower interval".
The margin of error is the range of values below and above the sample statistic in a confidence interval.
Normal distribution, is a "probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean".
Solution to the problem
In order to find the critical value we need to take in count that we are finding the interval for a proportion, so on this case we need to use the z distribution. Since our interval is at 95% of confidence, our significance level would be given by 
and 
. And the critical value would be given by:
Part a
The margin of error for the proportion interval is given by this formula:
(a)
And on this case we have that 
and we are interested in order to find the value of n, if we solve n from equation (a) we got:
(b)
Since we don't have prior information about the population proportion we can use ase estimator 
. And replacing into equation (b) the values from part a we got:
And rounded up we have that n=2401
Part b
For thi case the estimator for p is 
And rounded up we have that n=626
And we see that if we have previous info about p the sample size varies a lot.
