12. \(\frac{3x(x-5)}{(x+1)(x-5)}=\)

Сокращаем на \(x-5\): \(\frac{3x(x-5)}{(x+1)(x-5)}=\frac{3x}{x+1}\), \(x\ne-1,5\).

Ответ: \(\frac{3x}{x+1}\)