David wants to determine whether or not the number 73 is a prime number. He has already determined that the prime numbers, 2, 3, *EDIT (5), and 7 are not factors of 73. Does david need to test any other numbers to see if 73 is a prime number? (HINT: 7 x ? = 73)

I know there are various methods to deducing if a number is prime. I just know that 73 is. What method are they referring to in this problem? They want me to use it later in a different problem to determine if 437 is prime. What is so special about 7?