3 (10) Let \( A \in M_{n \times m}(\mathbb{R}) \), \( B \in M_{m \times n}(\mathbb{R}) \). Prove that if \( \lambda \) is an eigenvalue of \( AB \) and \( \lambda \neq 0 \) then \( \lambda \) is an eigenvalue of \( BA \).
3 (10) Let \( A \in M_{n \times m}(\mathbb{R}) \), \( B \in M_{m \times n}(\mathbb{R}) \). Prove that if \( \lambda \) is an eigenvalue of \( AB \) and \( \lambda \neq 0 \) then \( \lambda \) is an eigenvalue of \( BA \).