9 The cubic equation \(x ^ { 3 } + A x ^ { 2 } + B x + 15 = 0\), where \(A\) and \(B\) are real numbers, has a root \(x = 1 + 2 \mathrm { j }\).
- Write down the other complex root.
- Explain why the equation must have a real root.
- Find the value of the real root and the values of \(A\) and \(B\).