How to find eigenvalues of this matrix

Question

How to find eigenvalues of this matrix: $\left( \begin{array}{ c c } 2 & 0 & 0 \\ 0 & 2 & 4 \\ 0 & -1 & 2 \end{array} \right) $

ATTEMPT:

$(2-λ) [(2-λ)(2-λ) + 4] = 0$

Answer 1

Use the usual method

$$\det (A - \lambda I) = 0$$

that is

$$\begin{vmatrix}2-\lambda & 0 & 0\\ 0 &2- \lambda & 4 \\ 0 &-1 &2 - \lambda \end{vmatrix} = (2 - \lambda)\begin{vmatrix}2- \lambda & 4 \\ -1 &2 - \lambda \end{vmatrix} = (2 - \lambda)\Bigg((2 - \lambda)^2 + 4\Bigg) = 0$$

You can take it from here.

Answer 2

You must enclose the first $2-\lambda$ between brackets. Then equal to $0$ and solve.