#### Answer

-4x

#### Work Step by Step

We factor the expressions in order to cancel out common factors in the numerator and the denominator:
Step 1: $\frac{4x^{3}-4x}{1-x^{2}}$
Step 2: $\frac{4x(x^{2}-1)}{-1(x^{2}-1)}$
Step 3: $\frac{4x}{-1}$
Step 4: $-4x$