Hint
What is a prime number? If a number cannot be divided evenly by anything except itself and 1, then it is called prime number. Example for prime number: 7 is a prime number, because the only factors of 7 are 1 and 7 (1*7=7).
Example for not prime number: 6 is not a prime number, because 1*6=6 and 2*3=6. The factors for 6 are 1,2,3 and 6.
Example for not prime number: 9 is not a prime number, because 1*9=9 and 3*3=9. The factors for 9 are 1,3 and 9.