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

Answer: 
12x