I'd just scale up the left fraction by x to get a common denominator of x³, and then they can easily be added together.
]]>Ha, ha, ha, that was VERY VERY... VERY
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x / x^2 = x * x^-2
I'd then do the same with the next part of the sum, giving you b - ax * x^-3. Now all that is left is
(x * x^-2) + (b - ax * x^-3), since none of the values share similar powers, I think this is as far as you can go. (Don't take my word for it, I'm new to this too).
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again, I need the whole solution, because the answer is at the end of the book