$follow (A)=follow(S)$

$follow (S)=first (A)$

$first(A)=(a,\epsilon)$

replace the production $S\rightarrow bSA$ with $\epsilon$ we get $S\rightarrow bS, follow(S)=(\$,a)$

$follow (S)=first (A)$

$first(A)=(a,\epsilon)$

replace the production $S\rightarrow bSA$ with $\epsilon$ we get $S\rightarrow bS, follow(S)=(\$,a)$