We can only add like terms because it is like if you break a vase and you need to replace it before ur mom comes home they need to be exactly the same.
"Like terms" are terms whose variables (and their exponents such as the 2 in x2) are the same. In other words, terms that are "like" each other
only like terms can be combined. We combine like terms to shorten and simplify algebraic expressions, so we can work with them more easily.
hopefully it help
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.
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.