Simplifying Rational Powers

Bah, okay. (Doh)

I'm trying to find a common denominator right now for this part of the problem:

Don't I multiply the denominators together? Thus making it?

Now since I can subtract the numerators it means I get 9.

That ends up as the right answer for the exponent of this:

However, in the solution it baffles me because they got the following:

= 12x^8/6-5/6/y^4/3

It looks like they just multiplied it by two, but I didn't think that was how you got a common denominator...