b/
`(x+y)^3 +(x-y)^3 - 6x^2y`
`= x^3 + 3x^2y + 3xy^2 + y^3 + x^3 - 3x^2y + 3xy^2 - y^3 - 6x^2y`
`= (x^3 +x^3) + (3x^2y-3x^2y) +(3xy^2 +3xy^2) +(y^3-y^3) - 6x^2y`
`= 2x^3 + 6xy^2 -6x^2y`
c/
`(x-1)^3 +x(x+3)^2 +1`
`= x^3 - 3x^2 + 3x - 1 + x(x^2 +6x+9) +1`
`= x^3 -3x^2 +3x +x^3 + 6x^2+9x`
`= (x^3+x^3) +(-3x^2y+6x^2) +(3x+9x)`
`= 2x^3 + 3x^2 + 12x`