Negative Terms

Combining like terms is integer arithmetic on the numbers in front of the letter. $2k-9k$: 2 - 9 is -7, so the result is $-7k$. Taking away more k's than you have leaves a negative number of them, just as 2 - 9 is below zero.

The rules from Section 1 apply unchanged. $-x-4x$ is $-5x$, since -1 - 4 is -5. And $-6k+6k$ is $0k$, which is 0: the terms cancel.

A full example: $2k-9k+4$. The k terms give -7k and the number 4 stays alone, so the result is $-7k+4$. A negative first term is perfectly simple; there is no need to rearrange it.