If one of the roots of a cubic is a complex root, then so also is one other root.
That's because complex roots always come in pairs, as complex conjugates.
If one root is r1 = a + b.i, then the other (complex) root is r2 = a - b.i
a + b.i and a - b.i are called complex conjugates.
So, if you have been given one complex root, then simply take its complex conjugate as the other complex root.