We substitute both and simplify the equations:
9+3a+b=0
1-a+b=0
to solve the prior equations together we multiply the second equation by 3 and add it to the first one and we get:
12+3b=0
3b=-12
b=-12/3
We replace b with -4 and get the answer for a:
1-a-4=0
a=-3
