I think this 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 are no longer 0 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.