Răspuns :
[tex]\it 307 +10a \in M_9 \Rightarrow 306 + 1 +90 +a \in M_9 \Rightarrow \underbrace{306+9a}_{\in M_9}+a+1 \in M_9 \Rightarrow \\ \\ \Rightarrow a+1 \in \ M_9 \Rightarrow a\in\{8,\ 17,\ 26,\ 35,\ ...\}[/tex]
[tex]\it 307 +10a \in M_9 \Rightarrow 306 + 1 +90 +a \in M_9 \Rightarrow \underbrace{306+9a}_{\in M_9}+a+1 \in M_9 \Rightarrow \\ \\ \Rightarrow a+1 \in \ M_9 \Rightarrow a\in\{8,\ 17,\ 26,\ 35,\ ...\}[/tex]