What step have you made to go from (A+uU0) to (a+u)?Now try to write (A+uU0) as (a+u). You are almost there.
That was a hint. If you want to have u[MATH]\cdot[/MATH]1 inside the parenthesis , what should we do? Remember that a is a new non dimensional constant.What step have you made to go from (A+uU0) to (a+u)?