When you are asked to determine the minimum sample size required to estimate a population proportion (percentage) within a certain margin of error

n >= ((p*q)/d^2) * Z^2

where:

p is the proportion you want to estimate, and the sample proportion is given in the problem

q = 1-p

d = margin of error, usually given as a "+/-" percentage points

Z = z score corresponding to the desired level of confidence

Since you have no prior estimate of p, then use p = 0.5 (then q = 1-p = 0.5), d = .05, z = 2.326