If matrix a satisfies: AAT = E and | a | = - 1, then matrix a must have an eigenvalue of - 1. Why is it equal to proving that the determinant of | a + e | is 0

If matrix a satisfies: AAT = E and | a | = - 1, then matrix a must have an eigenvalue of - 1. Why is it equal to proving that the determinant of | a + e | is 0

To find the eigenvalue of a matrix, let the determinant | a - λ e | = 0 be obtained. Now | a + e | = 0 is equivalent to λ = - 1