Consider the following linear system.

• $x+2y-3z=a$
• $2x+3y+3z=b$
• $5x +9y-6z=c$

This system is consistent if $a, b$ and $c$ satisfy the equation

1. $7a-b-c=0$
2. $3a+b-c=0$
3. $3a-b+c=0$
4. $7a-b+c=0$