Complex eigenvalues of real matrices 
Given a matrix 
  $$A = \begin{pmatrix}
40  &  -29 & -11\\ \ -18  &  30\ & -12 \\\ \ 26 &24 & -50   \end{pmatrix}$$
  has a certain complex number $l\neq0$ as an eigenvalue. Which of the following must also be an eigenvalue of $A$:
  $$l+20, l-20, 20-l, -20-l?$$

It seems that complex eigenvalues occur in conjugate pairs. It is clear that the determinant of the matrix is zero, then $0$ seems to be one of the eigenvalues. 
Please suggest.

Hint: The trace of the matrix is $40+30+(-50)$.  As you observed, $0$ is an eigenvalue. 
