`a,(x-1)(x^2+x+1)-x(x+2)(x-2)=5`
`⇔x^3-1-x(x^2-4)=5`
`⇔x^3-1-x^3+4x=5`
`⇔4x=6`
`⇔x=3/2`
`b,(x-3)^3-(x-3)(x^2+3x+9)+9(x+1)^2=15`
`⇔x^3-9x^2+27x-27-x^3+27+9(x^2+2x+1)=15`
`⇔x^3-9x^2+27x-27-x^3+27+9x^2+18x+9=15`
`⇔45x=6`
`⇔x=2/15`
`c,x(x-5)(x+5)-(x-2)(x^2+2x+4)=3`
`⇔x(x^2-25)-x^3+8=3`
`⇔x^3-25x-x^3+8=3`
`⇔-25x=-5`
`⇔x=1/5`