Answer
(a) $f(-x)=-\frac{3x^3}{x^2-1}$
(b) $-f(x)=-\frac{3x^3}{x^2-1}$
Work Step by Step
(a) Replace $x$ with $-x$ to get $$f(-x)=\frac{3(-x)^3}{(-x)^2-1}=-\frac{3(-1)^3x^3}{(-1)^2x^2-1}=-\frac{3x^3}{x^2-1}$$ (b) Multiply $f(x)$ by $-1$ to get $-f(x)=-\frac{3x^3}{x^2-1}$.