rust, 300 bytes

Result

AC

Duration

994ms

Code [DL]

use std::io::Read;fn main(){let mut b=String::new();std::io::stdin().read_to_string(&mut b);let mut x:Vec<i32>=b.trim().split(|c|c<'!').map(|e|e.parse().unwrap()).collect();for i in 0..9{x[i]-=x[9+i%3]}print!("{}",(x[0]*(x[5]*x[7]-x[4]*x[8])+x[1]*(x[3]*x[8]-x[5]*x[6])+x[2]*(x[4]*x[6]-x[3]*x[7]))/6)}

stdin

08 02 03
19 21 86
67 03 66
50 77 89

stdout

44438

stderr

warning: unused `std::result::Result` which must be used
 --> /tmp/code.rs:1:53
  |
1 | use std::io::Read;fn main(){let mut b=String::new();std::io::stdin().read_to_string(&mut b);let mut x:Vec<i32>=b.trim().split(|c|c<'!').map(|e|e.parse().unwrap()).collect();for i in 0..9{x[i]-=x[9+i%3]}print!("{}",(x[0]*(x[5]*x[7]-x[4]*x[8])+x[1]*(x[3]*x[8]-x[5]*x[6])+x[2]*(x[4]*x[6]-x[3]*x[7]))/6)}
  |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_must_use)] on by default

strace