Exercise
How many positive integers of the form n2 - 9 are prime?
None
Infinitely many
Only one, namely 7
No, 7 = 42 - 9 is prime
No, for n > 4 we see that n - 3 is a proper divisor of n2 - 9
Indeed, for n > 4 we see that n - 3 is a proper divisor of n2 - 9, so n = 4 leads to the unique solution