依循 thepiano 老師的起頭發想，但改套用二項式定理，
\(8^{n+1}\equiv\left(7+1\right)^{n+1}\equiv C^{n+1}_1\cdot7^1+C^{n+1}_0\cdot7^0\equiv 7n+8\pmod{7^2}\)
\(\Rightarrow 8^{n+1}-8-7n\equiv 0\pmod{7^2}\)
\(\Rightarrow \left(8^{n+1}-8-7n\right)+7^2\equiv 0\pmod{7^2}\)
\(\Rightarrow 2^{3n+3}-7n+41\equiv 0\pmod{7^2}\)
