`a. x^2-y^2-2x+2y`
$=\left(x^2-y^2\right)-\left(2x-2y\right)$
$=\left(x-y\right)\left(x+y\right)-2\left(x-y\right)$
$=\left(x-y\right)\left(x+y-2\right)$
`b. 2x+2y-x^2-xy`
`=2(x+y)-x(x+y)`
`=(x+y)(2-x)`
`c. 3a^2-6ab+3b^2-12c^2`
$=3\left(a^2-2ab+b^2-4c^2\right)$
$=3\left[\left(a-b\right)^2-\left(2c\right)^2\right]$
$=4\left(a-b-2c\right)\left(a-b+2c\right)$
`d. x^2-25+y^2+2xy`
$=\left(x^2+2xy+y^2\right)-25$
$=\left(x+y\right)^2-5^2$
$=\left(x+y-5\right)\left(x+y+5\right)$
`e. a^2 + 2ab + b^2 - ac - bc`
`=(a^2 + 2ab + b^2) - (ac + bc)`
`=(a+b)^2- c(a+b)`
`=(a+b)(a+b-c)`
`g. x^2- 2x -4y^2- 4y`
`=(x-2y)(x+2y)-2(x+2y)`
`=(x+2y)(x-2y-2)`
