Use the formula 2n² to find the most electrons that the K, L, M and N shells can hold…

Step 1. (a) K: $2 \times 1^2 = 2$. L: $2 \times 2^2 = 8$. M: $2 \times 3^2 = 18$. N: $2 \times 4^2 = 32$.

Step 2. (b) $2 + 8 + 18 + 32 = 60$.

Step 3. (c) $2, 8, 9$ would make M the outermost shell with $9$ electrons. The outermost shell holds at most $8$, so the $19$th electron starts the N shell.