1)
x²-xy-4x+4y = (x-4)(x+y)

2)
ab-ac-bx+cx+c-b = (a-b)(x+c-1)

3)
a(a+3)-2(a+3) = (a-2)(a+3)

4)
ax-ay+5x-5y = (a+5)(x-y)

5)
x(x-y)+a(x-y) = (x+a)(x-y)

6)
2a-2b+ca-cb = (a+c)(2-b)

7)
2a-ac-2c+c² = (a-c)(c-2)

8)
bx+by-x-y-ax-ay = (b-a)(x-y)

9)
9x-6(x-1)-5(x+2) = 9x-6x+6-5x-10 = -2x-4

10)
7-4(3x-1) = 7-12x+4 = -12x+11

11)
2a(a+b-c)-2b(a-b-c)+2c(a-b+c) = 2a²+2ab-2ac-2ab+2b²+2bc+2ac-2bc+2c² = 2a²+2b²+2c²

12)
3x(x+y+c)-3y(x-y-c)-3c(x+y-c) = 3x²+3xy+3cx-3xy+3y²+3yc-3cx-3cy-3c² = 3x²+3y²-3c²