Answer :
[tex]x=amount\ she\ put\ in\ each\ week \\ 15x-9=21 \\ 15x=30 \\ x= \frac{30}{15} \\ x=2 \\ \boxed{2\ dollars\ each\ week}[/tex]
$2 because you add the 9 back to the 21 then divide by 15 to get 2