Remember, when you go to the store and buy 3 tomatoes and 6 onions, you do not have 9 tomonions. You can only combine like terms.
Like terms are terms that have exactly the same variable part.

Examples:
(3X2 + 5X + 3) + (2X2 -2X -5) = 5X2 +3X -2
 
(4X2 + 3X - 4) - (2X2 - 3X + 6) = 2X2+6X - 10