(a) It is given that in the expansion of \((4 + 2x)(2 - ax)^5\), the coefficient of \(x^2\) is \(-15\). Find the possible values of \(a\).

(b) It is given instead that in the expansion of \((4 + 2x)(2 - ax)^5\), the coefficient of \(x^2\) is \(k\). It is also given that there is only one value of \(a\) which leads to this value of \(k\). Find the values of \(k\) and \(a\).