Just need help with part 2

Reading proficiency: An educator wants to construct a 99.5% confidence interval for the proportion of elementary school children in Colorado who are proficient in reading. Part 1 of 3 (a) The results of a recent statewide test suggested that the proportion is 0.77. Using this estimate, what sample size is needed so that the confidence interval will have a margin of error of 0.07? A sample of 285 elementary school children is needed to obtain a 99.5% confidence interval with a margin of error of 0.07. Part: 1 / 3 Part 2 of 3 (b) Estimate the sample size needed if no estimate of p is available. A sample of elementary school children is needed to obtain a 99.5% confidence interval with a margin of error of 0.07