There are 2 approaches :
A) Reaching to final state B and then considering the loops on it..
In that case to reach state B we need : a*b.
And then incoming loops are because of 'a' and 'ba*b'..
So regular expression formed this way = a*b ( a + ba*b )*
B) Imagining A to be a final state considering loops on it and then reaching state B
In that case incoming loops on A are 'a' and 'ba*b'...
And then we reach B using : ba*..
So regular expression formed this way : (a + ba*b)* ba*..
The earlier approach is known as right resolution and later approach is known as left resolution..
Hence both regular expressions are equivalent...
In fact the property = > P(QP)* = (PQ)* P follows from this approach only..