Ithinkthis is what they are getting at... we have

And decide to let

Then so our integral becomes...

Notice that I have left the limits of integration out. They areno longer0 and 3. One way to prevent errors would just be to always explicitly write

But that's tedious.

So we will see that, when x = 0, u = 3 (from our choice of u sub above).

Likewise, when x = 3, u = ...

THOSE are the "A" and "B" to which they refer.