Answer:
a=-3, b=6
Explanation:
1) First, take a=1/2b-6 and plug it into 2a+3b=12
2(1/2b-6)+3b=12
2) Multiply out the 2
b-12+3b=12
3) Combine like terms
4b-12=12
4) Isolate b by adding 12 to the other side
4b=24
5) Divide by 4 so b doesn't have a variable in front
b=6
To solve for a:
You know that b=6, and a=1/2b-6 so you can plug in b to solve for a
a=1/2(6)-6
Solve the equation to get a
a=3-6
a=-3
Attached is a written explanation