Find the first 3 terms in the expansion of \(( 2 - x ) ^ { 6 }\) in ascending powers of \(x\).
Given that the coefficient of \(x ^ { 2 }\) in the expansion of \(\left( 1 + 2 x + a x ^ { 2 } \right) ( 2 - x ) ^ { 6 }\) is 48 , find the value of the constant \(a\).