в) 4x x 3x-12-x-4.

в) $$ \frac{4x}{3x-12} - \frac{x}{x-4} = \frac{4x}{3(x-4)} - \frac{x}{x-4} = \frac{4x - 3x}{3(x-4)} = \frac{x}{3(x-4)} $$

Ответ: $$ \frac{x}{3(x-4)} $$