Another way you can do this is to use the sum and product of the conjugate roots. This will allow you to get the quadratic factor
x2 - (sum of roots)x + (product of roots)
sum of roots = (3 - 4i) + (3 + 4i) = 6
product of roots = (3 - 4i)(3 + 4i) = 9 + 16 = 25
The quadratic factor is (x2 - 6x + 25)
So your function is going to be
f(x) = 2x(x2 - 6x + 25)
just as Mark obtained for you.