a - 3b = 9
a = b - 3
With our set of equations, we should substitute our variables with their values, and whichever answer equals (or cancels) out is the correct one.
(-9, -6);
-9 - 3(-6) = 9
-9 + 18 = 9
18 = 18
This is correct so far.
(-9, -6);
-9 = -6 - 3
-9 = -9
This is also correct!
From the work above, we can conclude that the set of equations (-9, -6) is the correct answer.
I hope this helps!
