`(3x-1)^3-(x-2)^3+26x(x^2-2)+21x(x+1)=25`

`=>27x^3-27x^2+9x-1-x^3+6x^2-12x+8+26x^3-52x+21x^2+21x-25=0`

`=>52x^3-34x-18=0`

`=>2(x-1)(26x^2+26x+9)=0`

`=>x-1=0`

`=>x=1`

Vậy `x=1`