1. Constraint on \( V \):
- The condition \( x_1 = x_2 \) imposes a single linear constraint on the elements of \( \mathbb{R}^4 \).
- This reduces the number of free variables from 4 to 3.
2. Basis for \( V \):
- A basis for \( V \) can be constructed by choosing three independent vectors that satisfy \( x_1 = x_2 \), such as:
\[
\{(1, 1, 0, 0), (0, 0, 1, 0), (0, 0, 0, 1)\}.
\]
3. Dimension of \( V \):
- The number of basis vectors is 3, so:
\[
\text{dim}(V) = 3.
\]