In Exercises $15-22,$ solve the given linear system by any method.

$(15.)$

$2x_1+x_2+3x_3=0$

$x_1+2x_2=0$

$x_2+x_3=0$

$(17).$

$3x_1+x_2+x_3+x_4=0$

$5x_1-x_2+x_3-x_4=0$

$2x-y-3z=0$

$-x+2y-3z=0$

$x+y+4z=0$

$v+3w-2x=$

$2u+v-4w+3x=$