a is given to us so just plug a into the first equation:
b-3 - 3b =9
Add 3 to both sides:
b-3b=12
Combine like terms:
-2b=12
Divide by -2 to get b by itself:
b=-6
The only answer with b as -6 is the first one, (-9,-6)
The first one, (-9,-6) because if you were to plug it in the answers would equal therefore, the equation is true.