To add expressions where many different variables are used, be sure to remember like terms. Like terms are terms which have the same variable bases and exponents. For instance a and b are different bases but ab and ab are the same. a^2b and ab^2 are different bases while a^2b^2 and a^2b^2 are the same. Both the variable and the exponents must be the same.
In this expression, there is 1 variable with three types of exponents. x^3, x^2, and x should be combined only with the same type.
