Answer:
Like terms are terms whose variables are the same.
We can only add them because if they does not have the same variable you do not actually know if the variables are equivalent and you will get the wrong answer.
Explanation:
Ex: 4z,7z,108z,45z
Ex: 4w-2h+7a-2w
You can not add 4w and 7a because you do not know what the variables equal so you do not know the true number so add or subtract by.