9x + 3x =
9x is a term. 9 is the number part and is called a coefficient. x is the letter part and is called a variable. When a coefficient is combined with a variable, a term is form: 9x
9x and 3x are considered like terms because they both have the same variable. Like terms can be combined by adding the coefficients of the terms. The variable does not change.
9x + 3x = 12x