`a) ( x+ 2y - 3z) ( x - 2y +3z )`
`=[( x+ 2y)- 3z][( x - 2y)+3z]`
`=(x+2y)^2-(3z)^2`
`=x^2+4xy+4y^2-9z^2.`
`b) (a + 1)(a+2) ( a^2 + 4) (a-1) (a^2 + 1) (a - 2)`
`=[(a+1)(a-1)(a^2+1)][(a+2)(a-2)(a^2+4]`
`=[(a^2-1)(a^2+1)][(a^2-4)(a^2+4)]`
`=(a^4-1)(a^4-16)`
`=a^8-16a^4-a^4+16`
`=a^8-17a^4+16.`