The question is:
What is the general form of the natural numbers that have :
a) a divisor beyond 1 and himself?
b) a number prime of divisors?
For letter a) I know that when a number has two divisors, 1 and himself we define him as prime otherwise he is composite. But I don't understand what form general this question is asking, how I can to write this? For letter b) I think that, if I can answer the letter a) I will answer the letter b).