Is this a subspace, if so, why and how?

1) set of all vectors from R^n which have a first coordinate of 0.

2) set of all vectors from R^2 of form

$\displaystyle \begin{bmatrix}x\\y \end{bmatrix} $ with x - y =2

3) set of all vectors from R^3 of form

$\displaystyle \begin{bmatrix}x\\2y-x\\x-y \end{bmatrix} $

4) set of all vectors $\displaystyle \begin{bmatrix}x1\\x2\\x3\\x4\\x5 \end{bmatrix} $ from R^5 satisfying x1 + 2x2 = x3 & x4=2x5