b) `9x^2 - (3x - 4)^2`
`= (3x)^2 - (3x - 4)^2`
`= (3x + 3x - 4) [3x - (3x - 4)]`
`= (6x - 4) (3x - 3x + 4)`
`= 4 (6x - 4)`
`= 24x - 16`
d) `(a^2 + 2a + 3) (a^2 + 2a - 3)`
`= (a^2 + 2a)^2 - 3^2`
`= (a^2 + 2a)^2 - 9`
f) `(y + 2z - 3) (y - 2z - 3)`
`= (y - 3)^2 - (2z)^2`
`= (y - 3)^2 - 4z^2`
g) `(3y + 4) (9y^2 - 12y + 16)`
`= (3y + 4) [(3y)^2 - 3y . 4 + 4^2]`
`= (3y)^3 + 4^3`
`= 27y^3 + 64`
j) `(x + y)^3 - (x - y)^3`
`= [(x + y) - (x - y)] [(x + y)^2 + (x + y) (x - y) + (x - y)^2]`
`= (x + y - x + y) (x^2 + 2xy + y^2 + x^2 - y^2 + x^2 - 2xy + y^2)`
`= 2y (3x^2 + y^2)`
`= 6x^2y + 2y^3`
