`a)`

`x^2-2x(3x+1)=-5(x^2+1)+21`

`<=>x^2-6x^2-2x=-5x^2-5+21`

`<=>(x^2-6x^2+5x^2)+(-2x)=-5+21`

`<=>-2x=16`

`<=>x=-8`

Vậy `x=-8`

`b)`

`(x-2)(x+5)=(x-3)^2+9x`

`<=>x^2+5x-2x-10=x^2-6x+9+9x`

`<=>(x^2-x^2)+(5x-2x+6x-9x)=9+10`

`<=>0x=19` (vô lý)

Vậy không tìm được giá trị `x` thỏa mãn.

`c)`

`2x(2x-7)-9=(2x+3)(2x-3)-14`

`<=>4x^2-14x-9=4x^2-9-14`

`<=>(4x^2-4x^2)+(-14x)=-9-14+9`

`<=>-14x=-14`

`<=>x=1`

Vậy `x=1`

`d)`

`(2x-1)(3x+1)=(3x+2)^2-3x^2`

`<=>6x^2+2x-3x-1=9x^2+12x+4-3x^2`

`<=>(6x^2-9x^2+3x^2)+(2x-3x-12x)=4+1`

`<=>-13x=5`

`<=>x=-5/13`

Vậy `x=-5/13`

`#owen`