We can add like terms (i.e. those terms with the same variables to the same powers) so we add 2x to 4x to get 6x.
Note that 6x and -6 are not like terms since the term -6 doesnot have a variable, so we cannot add them.
Thus the answer will be 6x-6.
** The question as originally written indicates an equation. Equations can be solved, while expressions can be simplified or evaluated. If this had been an equation, then there would be no unique answer, as the solution depends onthe value of x -- for each value of x the expression takes on a new value. **