x−4y+7z=g ...(i)
3y−5z=h ...(ii)
−2x+5y−9z=k ...(iii)
(i) + (ii), we get
x−y+2z=g+h ...(iv)
(ii) - (iii), we get
2x−2y+4z=h−k
⇒x−y+2z=2h−k ...(v)
Since, coefficient of x,y,z in (iv) and (v) are same. Hence, for system to be consistent
g+h=2h−k
⇒2g+2h=h−k
⇒2g+h+k=0