I'm stuck please solve

A researcher wishes to estimate the percentage of adults who support abolishing the penny. What size sample should be obtained if he wishes the estimate to be within 5 percentage points with 90% confidence if (a) he uses a previous estimate of 38%? (b) he does not use any prior estimates? Click here to view the standard normal distribution table (page 1). Click here to view the standard normal distribution table (page 2). (a) n= (Round up to the nearest integer.) (b) n = (Round up to the nearest integer.)