`1a)x^2-4=x^2-2^2=(x-2)(x+2)`
`b)(x+y)^2-(x-y)^2`
`=(x+y-x+y)(x+y+x-y)`
`=2y.2x`
`=4xy`
`c)(x-y)^3=x^3-3x^2y+3xy^2-y^3`
`d)3x^2:x=3x`
`2a)3x^2-3y^2-12x+12y`
`=(3x^2-3y^2)-(12x-12y)`
`=3(x^2-y^2)-12(x-y)`
`=3(x-y)(x+y)-12(x-y)`
`=(3x+3y-12)(x-y)`
`b)x^2-3x-4`
`=x^2-4x+x-4`
`=(x^2-4x)+(x-4)`
`=x(x-4)+(x-4)`
`=(x+1)(x-4)`
`c)2x+4y=2(x+2y)`
`d)x^2+2xy+y^2-1`
`=(x^2+2xy+y^2)-1^2`
`=(x+y)^2-1^2`
`=(x+y-1)(x+y+1)`
