Final answer:
The additive inverse of 6x-21 is -6x+21.
Step-by-step explanation:
The additive inverse of a number is the opposite of that number, when added to the original number, equals 0. To find the additive inverse of 6x-21, we simply change the sign of the entire expression. Therefore, the additive inverse of 6x-21 is -6x+21. When added together, 6x-21 and -6x+21 will equal 0.