11. \(\mathrm { f } ( x ) = ( a - x ) ( 3 + a x ) ^ { 5 }\), where \(a\) is a positive constant
- Find the first 3 terms, in ascending powers of \(x\), in the binomial expansion of
$$( 3 + a x ) ^ { 5 }$$
Give each term in its simplest form.
Given that in the expansion of \(\mathrm { f } ( x )\) the coefficient of \(x\) is zero,
- find the exact value of \(a\).