VGJ:let l=map(split(getline(1),' '),'str2nr')
:for i in range(9)
let l[i+3]-=l[i%3]
endfor
C=(l[3]*(l[7]*l[11]-l[8]*l[10])+l[4]*(l[9]*l[8]-l[11]*l[6])+l[5]*(l[6]*l[10]-l[7]*l[9]))/6
ZZ