crystal, 115 bytes

Result

AC

Duration

1833ms

Code [DL]

Y=`dd`.split;i=2;a,b,c,d,e,f,g,h,i=Y.map{|e|i+=1;e.to_i-Y[i%3].to_i}[3,9];p (a*e*i+b*f*g+c*d*h-a*h*f-b*i*d-c*g*e)/6

stdin

71 94 85
51 92 89
29 26 04
59 27 67

stdout

15270

stderr

0+1 records in
0+1 records out
36 bytes copied, 0.000110887 s, 325 kB/s

strace