If #x=1+i# is a factor of #ax^2+bx+c# then what is another factor?
I will assume that by 'factor' you intended 'zero' or 'root'.
A 'factor' would be something like
In general, given any polynomial with Real coefficients, any zeros are Real or occur in Complex conjugate pairs.
From the perspective of the Real numbers, there is no way to tell the difference between