I have been sweating buckets over my recent statistics test on probability. It's not coming to me, and my textbook isn't helping me much. I am basically clueless on how to calculate sample size with no preliminary estimate.

Here is the example I'm pouring over.

You are a travel agent and wish to estimate, with 95% confidence, the proportion of vacationers who plan to travel outside the United States in the next 12 months. Your estimate must be accurate within 3% of the true proportion. Find the minimum sample size needed if no preliminary estimate is available.

Any way I can get some help showing how to solve this problem? Any help is greatly appreciated.