Bad algebra. It's not (4x)^5, it's 4 (x)^5.

A good way to rewrite it is:

4^1 * x^5

So 3 / (4^1 * x^5) = 3*4^-1 * x^-5. Of course, this is just (3/4) x^-5.

Edit: Oh, and my point in my first post was that you should just take the fraction out before you integrate, then multiply. I forgot to say that though.