Multiply the rational expression

Simplify your work by factoring before multiplying.  We get:

4(3f-4)          5
---------- * -----------------
    16        (3f-4)(3f+4)

Cancel the (3f-4) terms, obtaining:

             20
------------------
       16 (3f+4)
                                    5
Reducing, we get -------------      and this is the desired result.
                               4(3f+4)