Solve what actually?
I see no equation or a problem. Only two strings of variables I suppose?
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.
So use "z^{-2}".NOTE: the exponents behind the z's are suppose to be -2.