How to determine if a constraint is redundant in a linear programming problem

Question

my TA told me that a constraint is redundant iff it is a linear combination of the other constraints. Let's consider these constraints:

1. $2x_1 + x_2 \leq 4$
2. $-2x_1 + x_2 \leq 2$
3. $x_1 - x_2 \leq 1$

Also $x_1,x_2 \geq 0$

If we draw a graph, we'll see that the polytope formed only depends on the 1st and 3rd constraint, so the second constraint is redundant. However, the second constraint isn't a linear combination of the other two, since

$ \begin{bmatrix} 2 & 1 & | & -2 \newline 1 & -1 & | & 1 \newline 4 & 1 & | & 2 \newline \end{bmatrix} $

is inconsistent. What am I misunderstanding?

edit: I made a mistake, the second constraint is not redundant