Let a, b be two non-zero real numbers. If p and r are the roots of the equation x^2-8 a x+2 a =0 and q and s are the roots of the equation x^2+12 b…

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