a,
`(3x-4)(x+9)=0`
`<=>`\(\left[ \begin{array}{l}3x-4=0\\x+9=0\end{array} \right.\) `<=>`\(\left[ \begin{array}{l}x=\dfrac43\\x=-9\end{array} \right.\)
Vậy `x \in {-9;4/3}`
b,
`(-12)^2 x=56+10.13x`
`<=>56+130x=144x`
`<=>14x=56`
`<=>x=4`
Vậy `x=4`
c,
`5(x-7)=7(x-3)`
`<=>5x-35=7x-21`
`<=>2x=-14`
`<=>x=-7`
Vậy `x=-7`
d,
`3(x-8)-6(x-5)=12`
`<=>3x-24-6x+30=12`
`<=>-3x+6=12`
`<=>-3x=6`
`<=>x=-2`
Vậy `x=-2`