Let \(a,b\) be two non-zero real numbers. If \(p\) and \(r\) are the roots of the equation \(x^{2} - 8ax + 2a = 0\) and \(q\) and \(s\) are the roots of the equation \end{enumerate} \(x^{2} + 12bx + 6b = 0\), such that \(\frac{1}{p},\frac{1}{q},\frac{1}{r},\frac{1}{s}\) are in A.P., then \(a^{- 1} - b^{- 1}\) is equal to