skill comes from experience. Just look for common factors.
2ax-6bx+ay-3by
You have a 2and a 6, and a 1 and a 3. So,
2x(a-3b)+y(a-3b)
(2x+y)(a-3b)
Now you try the others.
a) 2ax-6bx+ay-3by
b) 2ax-6bx-ay+3by
c) a^2b+c^2b-c^2b-a^2b
d) a^3+3a^2+2a+6
e) ax-2ay+2bx-4by
1 answer