What I did is,
Take S->aA;
then for A->aAA;
and from now on, to have minimum length string, put appropriate productions,
for 1st A, A->b and for 2nd A, A->bS;
and then S->bB; B->a; (here to have min string, I just use B ->a rather than bBB | aS, which led to longer string)
All the bold letters denote string, which is "aabbba".
Please correct me if I'm wrong and share your approach too!
