I'd use synthetic div. here, with 2 as my divisor:
________________
2 / 1 -11 18
2 -18
----------------------------
1 -9 0
Note that the coefficients of the missing factor are given here and are 1 and -9. Thus, (x-9) is the missing factor.
If this is supposed to be

Then this can simply be reduced to

by the double angle formula for cosine