7 In the binomial expansion of \(( k + a x ) ^ { 4 }\) the coefficient of \(x ^ { 2 }\) is 24 .
- Given that \(a\) and \(k\) are both positive, show that \(a k = 2\).
- Given also that the coefficient of \(x\) in the expansion is 128 , find the values of \(a\) and \(k\).
- Hence find the coefficient of \(x ^ { 3 }\) in the expansion.