(a)[4]
For the expansion of $(4 + 2x)(2 - ax)^5$, the coefficient of $x^2$ is $-15$. Find the possible values of $a$.
(b)[4]
Now suppose that in the expansion of $(4 + 2x)(2 - ax)^5$, the coefficient of $x^2$ is $k$. It is also stated that exactly one value of $a$ gives this value of $k$. Find the values of $k$ and $a$.