You can only add or subtract monomials that have the same variable and the same exponent. However, you can multiply and divide monomials with unlike terms.
Which of the following statements about math operations is incorrect?
all of these statements are correct 

you can multiply monomials that have different variables and different exponents 

you can subtract monomials that have the same variable and the same exponent 

you can add monomials that have the same variable and the same exponent 