In solving the equation (x + 4)(x – 4) = 9, Eric stated that the solution ...

In solving the equation (x + 4)(x – 4) = 9, Eric stated that the solution would be x + 4 = 9 => x = 5 or (x – 4) = 9 => x = 13 However, at least one of these solutions fails to work when substituted back into the original equation. Why is that? Please help Eric to understand better; solve the problem yourself, and explain your reasoning