Find a basis of the subspace of http://webwork2.asu.edu/webwork2_fil...8083707491.png consisiting of all vectors of the form

$\displaystyle \begin{bmatrix} x_1\\6x_1+x_2\\7x_1+3x_2\\-2x_1+6x_2\\ \end{bmatrix} $

i am not sure how to find this since there are variables $\displaystyle x_1 $ and $\displaystyle x_2 $

Thank you for any help :)