Computing the determinant of a $4\times4$ matrix 
Compute the determinant of \begin{vmatrix} 2 & -1 & 1 & 3 \\ 1 & 2 & 0 & 1 \\ 2 & 1 & 1 & -1 \\ -1 & 1 & 2 & -2 \end{vmatrix}

My try:
$$\begin{vmatrix} 2 & -1 & 1 & 3 \\ 1 & 2 & 0 & 1 \\ 2 & 1 & 1 & -1 \\ -1 & 1 & 2 & -2 \end{vmatrix}_{R_2\rightarrow2R_2-R_1\\R_3\rightarrow R_3-R_1\\R_4\rightarrow2R_4+R_1}$$
$$\begin{vmatrix} 2 & -1 & 1 & 3 \\ 0 & 5 & -1 & -1 \\ 0 & 2 & 0 & -4 \\ 0 & 1 & 5 & -1 \end{vmatrix}$$
Now I took $$\begin{vmatrix} 5 & -1 & -1 \\ 2 & 0 & -4 \\ 1 & 5 & -1 \end{vmatrix}=92$$
But the answer is $46$. Where did I go wrong?

You multiplied two rows by two, so your answer (184) is 4 times greater than the actual determinant. This video may help.
