[tex]^1) ~70 - 70 : a = 60 = \ \textgreater \ 70 : a = 70 - 60 = \ \textgreater \ 70 : a = 10 = \ \textgreater \ a = 70 : 10
= \ \textgreater \ \boxed{a = 7} = \ \textgreater \ \underline{S = 7}
^2) ~ 12 + 9 * a = 21 = \ \textgreater \ 9 * a = 21 - 12 = \ \textgreater \ 9 * a = 9 = \ \textgreater \ a = 9 : 9 = \ \textgreater \
\boxed{a = 1} = \ \textgreater \ \underline { S = 1 } [/tex]
