.
1.  This statement, as it presented in the post, might be TRUE and MIGHT be FALSE,
    DEPENDING on the polynomial f(x).
    For example,  if f(x) = 
, then (x-3) is the factor of f(x), but f(-3) = 
 = 
 = 36 is not zero.
    From the other side, if f(x) = (x-3)*(x+3), then (x-3) is the factor of f(x), and f(-3) = (3-(-3))*(3+(-3)) = 0.
2.  But, if I modify the statement in this way:
         Determine whether the statement is true or false.
              If x - 3 is a factor of a polynomial f(x), then f(3) = 0
    then this MODIFIED statement is TRUE.
    It follows from the "Remainder theorem".
On "Remainder theorem" see the lesson 
    - Divisibility of polynomial f(x) by binomial (x-a). The Remainder theorem
in this site.