Answer: 1/2 (choice A)
===========================================
Step-by-step explanation:
The two equations given to us are
Divide the second equation over the first equation and that would lead to b = 6
Notice how the 'a' terms divide to 1 and go away, i.e. cancel out.
The b terms divide to (b^2)/b = b
The right hand side values divide to 18/3 = 6
So that's how we end up with b = 6
-------------------------
Now if b = 6, then we can say,
ab = 3
a*6 = 3
a = 3/6
a = 1/2
Or we could say
ab^2 = 18
a*6^2 = 18
a*36 = 18
a = 18/36
a = 1/2