3.已知方程组 ) (x)_(1)+(x)_(2)+2(x)_(3)=a 3(x)_(1)-(x)_(2)-6(x)_(3)=a+2 (x)_(1)+4(x)_(2)+11(x)_(3)=a+3 .-|||-问a为何值时方程组有解?并在有解时求其通解.