Confidence intervals (sample size)

A political candidate has asked you to conduct a poll to determine what percentage of people support her.

If the candidate only wants a 2% margin of error at a 90% confidence level, what size of sample is needed?

I attempted the question as below:

2 a/2=1.96

and the EB= .02

The video I was watching shows to divide 1.96/.02 and square the result but when I try to enter 6765. my study guide shows the answer is incorrect. What is the correct answer and what part of my formula is incorrect I have a test coming up and I need to master the formula.

You want to obtain a sample to estimate how much parents spend on their kids birthday parties. Based on previous study, you believe the population standard deviation is approximately =31.4 dollars. You would like to be 90% confident that your estimate is within 5 dollar(s) of average spending on the birthday parties. How many parents do you have to sample? any adivice on this one?