Do a 2 sided hypothesis test where the alternative hypothesis is that the p's differ. Using the null hypothesis you should use the pooled estimator of P.
Then use the central limit theorem to get an approximate test statistic.
In a large city A, 20% of a random sample of 900 school boys had defective eye-sight. In another large city B, 18.5% of the random sample of 1600 school boys had the same defect. Is the difference between the two proportions significant?