-Combine all the like terms...
-If the equation is 4xy+17xy-12x, then you add 4xy and 17xy because they are like terms.
-So, you get 21xy-12x. You don't do anything else because there are no more like terms.
-If the equation is 3xy+6z-(7xy-12x), then you must first change the signs of the numbers within the parenthesis.
-So, the equation becomes 3xy+6z-7xy+12x.
-Combine all the like terms, and you get -4xy+6z-12x.