When combining like terms how do you know to add or subtract?

It’s depends on the sign.

Step-by-step explanation:

For example if your question is 3x+16y-8-7x+8y then the answer would be -4x+24y-8 if they have the same variable look at them as a separate equation.

"Like Terms" means that you can add or subtract two terms. For instance, you know that you can add and get 5. You were able to add these two 'terms' ( the '2' and the '3') because they are both numbers! However, you might also know that you cannot 'combine' 2 and x. Since 2 is a number and 'x' is not, they are not like terms.

Basically if its two numbers that are the same, For example (-2x and -4x) and have with the same variables you would add them. If they are like this (5x and -2x) you would subtract them because its a negative and positive number and get ( 3x).

Hope i helped