`16) x^2-2xy+y^2-16z^2`

`= (x^2-2xy+y^2)-16z^2`

`= (x-y)^2 - (4z)^2`

`= (x-y-4z)(x-y+4z)`

`17) 8x^2-16x+8-32y^2`

`= 8(x^2-2x+1-4y^2)`

`= 8[(x^2-2x+1)-4y^2]`

`= 8[(x-1)^2-(2y)^2]`

`= 8(x-1-2y)(x-1+2y)`

`18) 4x^2-4x+1-81a^2`

`= (4x^2-4x+1)-81a^2`

`= (2x-1)^2-(9a)^2`

`= (2x-1-9a)(2x-1+9a)`

`19) x^2-6xy+9y^2-121`

`= (x^2-6xy+9y^2)-121`

`= (x-3y)^2-11^2`

`= (x-3y-11)(x-3y+11)`

`20) 12x^2-24x+12-3y^2`

`= 3(4x^2-8x+4-y^2)`

`= 3[(4x^2-8x+4)-y^2]`

`= 3[(2x-2)^2-y^2]`

`= 3(2x-2-y)(2x-2+y)`

`21) a^2+6a-b^2+9`

`= (a^2+6a+9)-b^2`

`= (a+3)^2-b^2`

`= (a+3-b)(a+3+b)`

`22) x^3-4x^2-25xy^2+4x`

`= x(x^2-4x-25y^2+4)`

`= x[(x^2-4x+4)-25y^2]`

`= x[(x-2)^2-(5y)^2]`

`= x(x-2-5y)(x-2+5y)`

`23) x^2-y^2+14x+49`

`= (x^2+14x+49)-y^2`

`= (x+7)^2-y^2`

`= (x+7-y)(x+7+y)`