Ok, so knowing where to start is the main point:  Do parentheses and exponents first - let's take the top half (numerator):

Let's re-write the top (numerator) 3a(b-4) as (3a)(b-4)(b-4)  - this simply shows that (b-4)² = (b-4)(b-4), just like (2)(2) = 2²

The bottom (denominator) is 3a(b-4), can also be written as (3a)(b-4)

Now we have [(3a)(b-4)(b-4)] / [(3a)(b-4)].   (3a) / (3a) = 1, and (b-4) / (b-4) also = 1, so we're left with only one of the original (b-4) terms, and the answer is b-4