Find the sum of eigenvalues of the matrix: \( \begin{bmatrix} 4 & 1 \\ 3 & 6 \end{bmatrix} \)
For the matrix, $A = \begin{bmatrix} -4 & 0 \\ -1.6 & 4 \end{bmatrix}$, the eigenvalues ($\lambda$) and eigenvectors ($X$) respectively are: