Concept:
The dimension of a subspace is the number of free parameters needed to describe its vectors.
Given,
\[
W=\{(a,b,c):a+b+c=0\}
\]
This is a subspace of \(V^3\).
Step 1: Use the condition.
\[
a+b+c=0
\]
So,
\[
c=-a-b
\]
Step 2: Write general vector of \(W\).
\[
(a,b,c)=(a,b,-a-b)
\]
\[
(a,b,-a-b)=a(1,0,-1)+b(0,1,-1)
\]
Step 3: Count independent parameters.
The vector depends on two arbitrary parameters:
\[
a,\quad b
\]
Therefore,
\[
\dim W=2
\]
Step 4: Final answer.
\[
\boxed{2}
\]