Let
\(
v_1 =
\begin{bmatrix}
1 \\
1
\end{bmatrix}
\)
and
\(
v_2 =
\begin{bmatrix}
0 \\
1
\end{bmatrix}
\).
Let \(M\) be the matrix whose columns are
\(v_1,\; v_2,\; 2v_1 - v_2,\; v_1 + 2v_2\) in that order.
Then the number of linearly independent solutions of the homogeneous system
\(Mx = 0\) is ...........