`(a + b)^{3} - (a - b)^{3}`
`= [(a + b) - (a - b)] . [(a + b)^{2} + (a + b) . (a - b) + (a - b)^{2}]`
`= (a + b - a + b) . (a^{2} + 2ab + b^{2} + a^{2} - b^{2} + a^{2} - 2ab + b^{2})`
`= [(a-a) + (b + b)] . [(a^{2} +a^{2} + a^{2}) + (b^{2} - b^{2} + b^{2}) + (2ab - 2ab)]`
`= 2b . (3a^{2} + b^{2})`