Find the relationship between a,b, and c for which the system of equations will be consistent.

2x+y-z=a

x+2y+z=b

5x+4y-z=c

I need help for this, i can't seem to get the answer that the book has in the back which is, 2a+b-c=0. Can someone plzz help me solve this, or show me step by step on how to do this. Thanks!!