In subtracting polynomials, change the sign of each term of the subtrahend, then proceed to addition rule (combining like terms, considering the rule in addition of integers.
Note: I've changed the sign of each term of the the subtrahend.
1) 2a + b - 3c
-3a -2b -4c
-a -b -7c
2) 5x + 2y -6z
-7x + 2y -3z
-2x + 4y -9z
3) 7p - 3k + 4m
-8p + 4k + 2m
-p + k + 6m
4) 5x - 7y + 4z
-2x + 8y - 2z
3x + y + 2z
5) 9ax - 4ay - 8az
-2ax + 5ay + 9az
7ax + ay + az
(Note: Addition of polynomials was correctly answered above by the moderator)