Write down the first four terms of the expansion, in ascending powers of \(x\), of \(( a - x ) ^ { 6 }\).
Given that the coefficient of \(x ^ { 2 }\) in the expansion of \(\left( 1 + \frac { 2 } { a x } \right) ( a - x ) ^ { 6 }\) is - 20 , find in exact form the possible values of the constant \(a\).