Let f(x) be a differentiable function at x= a with f^′( a )=2 and f( a )=4. Then lim_x → a (x f( a )- a f(x))/(x- a) equals:

Let $\displaystyle f(x)$ be a differentiable function at $\displaystyle x=\mathrm{a}$ with $\displaystyle f^{\prime}(\mathrm{a})=2$ and $\displaystyle f(\mathrm{a})=4$. Then $\displaystyle \lim _{x \rightarrow \mathrm{a}} \frac{x f(\mathrm{a})-\mathrm{a} f(x)}{x-\mathrm{a}}$ equals: