Tìm x , biết : a) 2.(x+1)-4x=15 b) (x-1).(x+1)-x.(x+4) d) x mũ 2 – 6x = 9 Giúp tui zớii Vote 5 sao + ctlhn ạ

a) $2(x+1)-4x=15$

$↔2x+2-4x=15$

$↔-2x=13$

$↔x=-\dfrac{13}{2}$

b) $(x-1)(x+1)-x(x+4)$

$=x^2-1-x^2-4x$

$=-1-4x$

c) $x^2-6x=9$

$↔x^2-6x+9-9=0$

$↔(x-3)^2=9$

$\begin{cases}x-3=3\\x-3=-3\end{cases}$

$\begin{cases}x=6\\x=0\end{cases}$