False because u can only add them together if they are like terms (they are similar to eachother) in this case you can only add 3a + 9a together = 12a because the other numbers do not have a at the end therefore they are not similar. It doesn’t just have to be the letter a it can be any other letter or even indices e.g squared etc.
So just add the 3 and 7 = 10
12a + 10