[ 1 2 -2 -1]

[ 1 3 1 10]

[ 2 6 2 20]

Divide the third row by 2,

[ 1 2 -2 -1]

[ 1 3 1 10]

[ 1 3 1 10]

Subtract 2nd from 3rd and subtract 1st from 2nd

[ 1 2 -2 -1]

[ 0 1 3 11]

[ 0 0 0 0]

Multiply 2nd by (-2) add to 1st,

[1 0 -8 -23]

[0 1 3 11]

Thus,

x-8z=-23

y+3z=11

Solve,

x=8z-23

y=-3z+11

And "z" can be anything thus z=c

Thus,

(x,y,z)=(8c-23,-3c+11,c)