<s>[INST] Using EigenValues to form a diagonal matrix After going through my Linear algebra note, I know if for any matrix $A$, we find the eigenvalues and eigenvectors , we can construct a matrix P, such that $P^{-1}AP$ is a diagonal matrix.
Now for this matrix for example
$\left( \begin{array}{ccc}
1 & -2 \\
1 & 4 \end{array} \right)$
I found that that the EigenValues are $3,2$ and the eigenvectors are;
$\left( \begin{array}{ccc}
1 \\
-1   \end{array} \right)$
, $\left( \begin{array}{ccc}
1 \\
-2   \end{array} \right)$
so now using these eigenvectors as columns, I construct matrix $P$;
$\left( \begin{array}{ccc}
1 & 1 \\
-1 & -2 \end{array} \right)$ and find that $P^{-1}$ is $\left( \begin{array}{ccc}
-1 & -1 \\
2 & 1 \end{array} \right)$.
So going by the examples online and my lecture notes $P^{-1}AP$ should give me a diagonal matrix with the diagonal entries being the eigenvalues. 
Like this $\left( \begin{array}{ccc}
2 & 0 \\
0 & 3 \end{array} \right)$.
But when I multiply the matrices Ive got :
$P^{-1}AP$=$\left( \begin{array}{ccc}
-1 & -1 \\
2 & 1 \end{array} \right)$
$\left( \begin{array}{ccc}
1 & -2 \\
1 & 4 \end{array} \right)$  $\left( \begin{array}{ccc}
1 & 1 \\
-1 & -2 \end{array} \right)$ = $\left( \begin{array}{ccc}
2 & 0 \\
3 & 3 \end{array} \right)$.
Can anyone guide me to where I went wrong ? 
 [/INST] It was noted in the comments that the eigenvector for $2$ is wrong and the OP needs further help in finding an eigenvector for $2$.
Let $\begin{pmatrix} v_1\\ v_2\end{pmatrix}$ be an eigenvector for $2$. It holds that $\begin{pmatrix} 1 & -2\\ 1 & 4\end{pmatrix}\begin{pmatrix} v_1\\ v_2\end{pmatrix}=2\begin{pmatrix} v_1\\ v_2\end{pmatrix}$ and this is equivalent to $\begin{cases} v_1-2v_2&=2v_1\\ v_1+4v_2&=2v_2\end{cases}$, which in turn is equivalent to $v_1+2v_2=0$ (or $v_1=-2v_2$). Since $v$ is an eigenvector, it is not null, so take for instance $v_2=1$ to get $\begin{pmatrix} -2\\ 1\end{pmatrix}$.
</s>