(2 / x^4) - (4 / x^6)
We want the bottoms of each fraction to match.
We want the x^4 to be x^6, so we multiply the top and bottom of that fraction by x^2:
(2x^2 / x^6) - (4 / x^6)
The bottoms of the fraction are the same so we can now do this:
(2x^2 - 4) / x^6
We can factor out a 2 on the top:
2 (x^2 -2) / x^6