MHB What is the Derivative of xf(x) at x=4 Using the Product Rule?

[musad]

I am having trouble getting started with this question.
Suppose that f(4)=7 and f′(4)=−2. Use the product rule to find the derivative of xf(x) when x=4. Thanks

 

[Prove It]

I am having trouble getting started with this question.
Suppose that f(4)=7 and f′(4)=−2. Use the product rule to find the derivative of xf(x) when x=4. Thanks

It says exactly what to do, use the product rule on $\displaystyle \begin{align*} x\,f(x) \end{align*}$, you should get

$\displaystyle \begin{align*} \left[ x\,f(x) \right] ' &= x' \,f(x) + x\,f'(x) \\ &= 1\,f(x) + x\,f'(x) \\ &= f(x) + x\,f'(x) \end{align*}$

so what do you get when x = 4?