\(\dfrac{-21}{24}:\dfrac{-7}{3}=\dfrac{-21}{24}\cdot\dfrac{-3}{7}=\dfrac{-3}{8}\cdot\dfrac{-1}{1}=\dfrac{3}{8}\)

`2xx x+15=75`

`2 xx x=75-15`

`2 xx x=60`

`x=60:2`

`x=30`

:v

Sửa 

\(a,\\ \left(12x^2-12x^2\right)+\left(-4x+30x+6x-24x\right)-\left(10-12\right)=1\\ 6x+2=1\\ 6x=-3\\ x=-\dfrac{3}{8}\\ x=-\dfrac{1}{8}\)

\(b,\left(x^2-x^2\right)-\left(3x-2x-4x\right)-6=3\\ 3x-6=3\\ 3x=9\\ x=3\)

`b)`

\(2\left(3x-1\right)\left(2x+5\right)-6\left(2x-1\right)\left(x+2\right)=1\\ \left(6x-2\right)\left(2x+5\right)-\left(12x-6\right)\left(x+2\right)=1\\ 2x\cdot\left(6x-2\right)+5\left(6x-2\right)-x\left(12x-6\right)-2\left(12x-6\right)=1\\ 12x^2-4x+30x-10-12x^2+6x-24x+12=1\\ \left(12x^2-12x^2\right)-\left(4x-3x-6x+24x\right)-\left(10-12\right)=1\\ 19x+2=1\\ 19x=-1\\ x=-\dfrac{1}{19}\)

`c)`

\(\left(x+2\right)\left(x-3\right)-x\left(x-4\right)=3\\ x\cdot\left(x-3\right)+2\cdot\left(x-3\right)-x^2+4x=3\\ x^2-3x+2x-6-x^2+4x=3\\ \left(x^2-x^2\right)-\left(3x-2x+x-4x\right)-6=3\\ 2x-6=3\\ 2x=9\\ x=\dfrac{9}{2}\)