Find a basis of the subspace of ${\mathbb R}^4$ consisting of all vectors of the form $$\left\lbrack \begin{array}{c} x_1 \\ 3 x_1 + x_2 \\ 3 x_1 + 9 x_2 \\ 3 x_1 + 5 x_2 \end{array} \right\rbrack$$
Your answer should be a list of row vectors separated by commas. (Click vector to learn about entering vectors.)

Basis = $\big\lbrace$ $\big\rbrace.$