6. Given that the domain of the function $f(x)$ is $(-\infty, 0)$ $\cup(0,+\infty)$, and it satisfies $f\left(\frac{1}{x}\right)+\frac{1}{x} f(-x)=4 x$. Then the value of $f(2) f\left(-\frac{1}{2}\right)$ is ( ).
(A) 31.5
(B) 30.5
(C) -30.5
(D) -31.5
6. D.

Substitute $x=-2, x=\frac{1}{2}$ into the given equation, we get $f(2) f\left(-\frac{1}{2}\right)=-31.5$.