Question 1029945
{{{f(x) = k(x-2)(x-4)^2}}} for some undetermined constant k.

Now {{{f(1) = k(1-2)(1-4)^2 = -18}}} ==> -9k = -18 ==> k = 2.

{{{f(x) = 2(x-2)(x-4)^2}}} is the polynomial function of degree 3.  (Just do the necessary expansion.)