Answer is :3
S-->aB S-->aaB (if we use this production B-->aB) S-->aaEps (if we use this production B-->Eps) (Backtrack)
S-->aB S-->aEps (if we use this production B-->Eps) (Backtrack)
S-->aAb S-->abAbb (if we use this production) A-->bAb (Backtrack)
S-->aAb S-->aab (if we use this production A-->a) (Finally No Backtracking Here)