node, 151 bytes

Result

WA

Duration

579ms

Code [DL]

let[,,,a,b,c,d,e,f,g,h,i]=(""+require("fs").readFileSync(0)).split(/\s/).map((e,i,x)=>e|0-x[i%3]);
console.log((a*e*i+b*f*g+c*d*h-a*f*h-b*d*i-c*e*g)/6)

stdin

22 02 61
11 07 34
50 46 07
81 15 10

stdout

-146.66666666666666

stderr

strace