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Find an orthonormal basis of the plane $x_1 + 3 x_2 - x_3 = 0$ .

$\Bigg\lbrace$ $\left[\Rule{0pt}{3.6em}{0pt}\right.$ $\left]\Rule{0pt}{3.6em}{0pt}\right.$ , $\left[\Rule{0pt}{3.6em}{0pt}\right.$ $\left]\Rule{0pt}{3.6em}{0pt}\right.$ $\Bigg\rbrace.$