Concept:
A homogeneous system of linear equations always has at least the trivial solution.
If the equations are dependent, then the system has infinitely many solutions.
Step 1: Write the equations.
\[
2x+y+z=0
\]
\[
y-z=0
\]
\[
x+y=0
\]
Step 2: Solve using equations.
From
\[
y-z=0
\]
we get
\[
y=z
\]
From
\[
x+y=0
\]
we get
\[
x=-y
\]
Substitute in first equation:
\[
2(-y)+y+y=0
\]
\[
-2y+y+y=0
\]
\[
0=0
\]
So the first equation is dependent on the other two.
Step 3: Express solution.
Let
\[
y=t
\]
Then,
\[
x=-t,\qquad z=t
\]
So solutions are
\[
(x,y,z)=(-t,t,t)
\]
where \(t\in R\).
Thus there are infinitely many solutions.
Step 4: Final answer.
\[
\boxed{\text{An infinite number of solutions}}
\]