A political candidate has asked you to construct a poll to determine what percentage of people support her. If the candicate only wants a 5% margin error at a 90% confidence level , what size of sample is needed?

I believe that z = 1.64485 , and I would normally use n = ( z*o/ margin error)^2 to solve this problem ; is there another formula that I can use?....... Could I get some help. thanks in advance!!