Because 
has real coefficients, you know the complex root occurs along with its conjugate. That is, both 
are roots to 
.
This means that dividing through by both factors yields another polynomial with no remainder:
This means the last root (there are only three according to the fundamental theorem of algebra) is
.
This means that dividing through by both factors yields another polynomial with no remainder:
This means the last root (there are only three according to the fundamental theorem of algebra) is