(a) In the expansion of

\(\left(2k-\frac{x}{k}\right)^5,\)

where \(k\) is a constant, the coefficient of \(x^2\) is \(160\). Find the value of \(k\).

(b)

(i) Find, in ascending powers of \(x\), the first 3 terms in the expansion of \((1+3x)^6\), simplifying the coefficient of each term.

(ii) When \((1+3x)^6(a+x)^2\) is written in ascending powers of \(x\), the first three terms are \(4+68x+bx^2\), where \(a\) and \(b\) are constants. Find the value of \(a\) and of \(b\).