I don't understand 6k-6k+8-12k
Okay, so first, take out all the ones with variables. You should have 6k,-6k,and -12k. Then, you simply combine like terms. 6k-6k-12k is -12k, and you include the +8 in your answer, so the answer is -12k+8.
6k - 6k + 8 - 12k8 - 12k