6. When \(( 1 + a x ) ^ { n }\) is expanded as a series in ascending powers of \(x\), the coefficients of \(x\) and \(x ^ { 2 }\) are - 6 and 27 respectively.
- Find the value of \(a\) and the value of \(n\).
- Find the coefficient of \(x ^ { 3 }\).
- State the set of values of \(x\) for which the expansion is valid.