Something so different from what I taught. I have no idea how to solve this.
Did you notice that the x and z dependence of the first two terms are exactly the same? Combine them. If you were told "put the answers in descending order with regard to the x variable", since all three terms have , they can be in any order
x- 3x= -2x.
NOTE: the exponents behind the z's are suppose to be -2.