`~rai~`
\(C_x^1+6C_x^2+6C_x^3=9x^2-14x\quad(x\ge 3)\\\Leftrightarrow \dfrac{x!}{(x-1)!}+6.\dfrac{x!}{2!(x-2)!}+6.\dfrac{x!}{3!(x-3)!}=9x^2-14x\\\Leftrightarrow \dfrac{x(x-1)!}{(x-1)!}+6.\dfrac{x(x-1)(x-2)!}{2(x-2)!}+6.\dfrac{x(x-1)(x-2)(x-3)!}{6(x-3)!}-9x^2+14x=0\\\Leftrightarrow x+3x(x-1)+x(x-1)(x-2)-9x^2+14x=0\\\Leftrightarrow x+3x^2-3x+x^3-3x^2+2x-9x^2+14x=0\\\Leftrightarrow x^3-9x^2+14x=0\\\Leftrightarrow x(x-2)(x-7)=0\\\Leftrightarrow \left[\begin{array}{I}x=0\quad\text{(loại)}\\x=2\quad\text{(loại)}\\x=7\quad\text{(nhận)}\end{array}\right.\\\text{Vậy }x=7.\)
