Subtracting expressions with radicals

Solution 1:

Just view $ab^2\sqrt{2ac}$ as a separate entity.

As a matter of fact, let us use $d$ to represent it.

Then, we would have $120d-28d$, which is clearly $92d$.

Solution 2:

Observe that $\;120-28=92\;$ , and thus you have an expression of the form

$$120 K-28 K=92K\,.\;\text{In this case, we simply have}\;\;K=ab^2\sqrt{2ac}\;$$

Solution 3:

Although this question has several good answers and good points in the comments, I want to point out the explicit connection to the distributive law, in terms that can help in elementary school classrooms too.

Kids have little trouble answering the question

If I have 7 apples and get 2 more how many do I have then?

If you want to write the answer with more formal mathematics (nowadays called a number sentence) it's

7 apples + 2 apples = 9 apples .

This principle helps kids master place value: think "hundreds" instead of "apples" and you get $700 + 200 = 900$, even $900 + 200 = 1100$.

You can even touch on the etymology of "ninety" as coming from "nine tens".

In the OP's question the unit quantity of which there are first 128 and then only 92 is $ab^2\sqrt{2ac}$.