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
