By the rational zero theorem
This should help you: The Factor Theorem
You're not responding so I'll take that to mean that you need an extra push.
The rational zero theorem basically states that a polynomial of the form has rational zeros, then they must be given by the quotient of the divisors of divided by the divisors of .
So, begin by listing all of the divisors of the constant term:
Then list the divisors of the leading coefficient:
So, therefore, the possible zeros of your are .
So, now you can test these values by substition, or synthetic division. Now understand that (by the factor theorem as stroodle posted) if x=c is a zero of f(x), then is a factor of f(x).