powershell, 179 bytes

Result

WA

Duration

1232ms

Code [DL]

$a =(Read-Host).Split()
$b =(Read-Host).Split()
$c =(Read-Host).Split()
$d =(Read-Host).Split()
foreach($i in 0..2){
$b[$i]-=$a[$i]
$c[$i]-=$a[$i]
$d[$i]-=$a[$i]
}
echo(($b[0])/6)

stdin

20 66 06
03 97 29
99 40 71
86 55 48

stdout

20 66 06
03 97 29
99 40 71
86 55 48
-2.83333333333333

stderr

strace