The first three terms, in ascending powers of \(x\), in the expansion of \((2+ax)^n\) are \(1024-1280x+bx^2\). Find the values of \(n\), \(a\), and \(b\).

Hence find the term independent of \(x\) in the expansion of \((2+ax)^n\left(x-\dfrac1x\right)^2\).