By the semantics, Aphi holds in a state if all infinite paths leaving that state satisfy phi. A variable like b is a state formula, and state formulas hold on an infinite path iff they hold on the first state of that path. Hence, Ab holds in a state, if that state does not have outgoing infinite paths at all, or if b holds in that state.
With this, we could now see that E[1 SU (Ab)] is equivalent to E[1 SU b], and we have the following
E[1 SU (Ab)]
= E[1 SU (b | A false)]
= E( [1 SU b] | [1 SU (A false)])
= E[1 SU b] | E[1 SU (A false)]
= E[1 SU b]
In the last step, you have to understand that E[1 SU (A false)] is false: E demands that there must be an infinite path where 1 holds until a state is found on that path where A false holds. However, A false holds in states that have no infinite outgoing paths, and therefore this formula cannot hold on a state on an infinite path.
We can then continue with
EXXFGE[1 SU b]
= EXXFGEF b
= EX EX EF EG EF b