6.求解下列齐次线性方程组.-|||-(3) 2x1 +3x2 -x3-7x4 =0, 3x1 +x2+2x3 -7x4 = 0, 4x1 +x2-3x3+6x4=0, x1 -2x2+5x3-5x4=0.