[tex]x + y + z = 20709 \\ x = 3y \\ y = 2z[/tex]
[tex]3y + y + z = 20709 \\ y = 2z \\ 4y + z = 20709 \\ y = 2z \\ 4y + z = 20709 \\ y - 2z = 0[/tex]
[tex]4y + z = 20709 \\ 4y + 8z = 0 \\ 9z = 20709 \\ z = 2301[/tex]
[tex]y - 2 \times 2301 = 0 \\ y = 4602[/tex]
[tex]x = 3 \times 4602 \\ x = 13806[/tex]