I think you might be confusing the issue a little by substituting z for x, as x is the original variable in the function. Plato's way makes it easier, as you can pull the 6 out in front of the integral, then evaluate using the exponent rule for integrals.
When you divide a variable with an exponent by the same variable with a different exponent you subtract the exponent of the denomintor from the numerator.
So
= x^(2-1) = x
These math tags are killing me...how do i properly show that an exponent is ^(2-1), it doesnt display like this :P