Subtracting Results From Two Different Queries.

Hi, here's the situation:

I have a query A, that gives me the a set of numbers.
I have a query B, that gives me another set of numbers.

Numbers in B are always equal or larger than those in A. (B >= A)

I want to create a new query C, that does the subtraction. (C = B-A)

Here's an example to explain what I want.
Let's say I have A=4,7 and B=6,9

Now, when I create the query, I get the following result:
A | B | C=B-A
4 | 6 | 2
4 | 9 | 5
7 | 6 | -1
7 | 9 | 2

What I want is this:
A | B | C=B-A
4 | 6 | 2
7 | 9 | 2

How do I do that?

Thanks.