Can someone me? i think there's something wrong with this exercice (or else i'm doing it wrong).

Note that when adding or subtracting, you can only combine like terms. However, when multiplying and dividing, you can combine un-like terms, as long as they follow the rules.

These terms cannot be combined because they do not have the same variables, and you are adding.
7x + 3 = 7x + 3

These terms can be combined, because they have the same number of variables, "x".
7x + 3x = 10x

These terms can also be combined because they have the same amount of variables
7x - 3x = 4x

These terms can be combined because you are multiplying
7x(3) = 21x

These terms can be combined because they have the same amount of variables.
5x + 5x = 10x

These terms can be combined because you are multiplying. Note that when you multiply one x with another x, you get x²
7x(3x) = 21x²

hope this helps