How do I go about tackling this problem?

(I made a problem-modifying typo in the first)

The American Medical Association wishes to determine the percentage of obstetricians who are considering leaving the profession becaose of the rapidly increasing number of lawsuits against obstetricians. How large a sample should be taken to find the answer to within + or - 3% at the 95% confidence level?

You would need to know the number of obstetricians. Say there was only 2 Drs or 10,000