Moderate -0.3 This is a straightforward proof by induction with a simple divisibility statement. The base case is trivial, and the inductive step requires only basic algebraic manipulation (factoring out 7 and recognizing that 7≡3(mod 4) gives 7^k+3^{k-1}≡3·3^k+3^{k-1}≡4·3^{k-1}). While it requires understanding the induction framework, the arithmetic is routine and the problem follows a standard template, making it slightly easier than average.