Posted by paula on Sunday, November 22, 2009 at 8:08pm.
The basic rule is, you need to know the divisibility tests for the first few primes, such as 2,3,5,(7,11,13).
For 392, start by dividing by 2 until the number is odd.
392
=2*196
=2*2*98
=2*2*2*49
Since you know 49=7*7, you would have no problem completing the prime factorization.