Actually, just did it again and got (D). However, it's an SAT question and I need the fastest way to solve it... what I did was plug in values for a and b (3 and 2, respectively) then find c (78) and plug a (3) back into each of the choices to see if c (78) and the answer were equal. Is there a faster way to do it?
1/3c = 6b^2, so
c = 18b^2
Now, 3b=2a, so
18b^2 = 2(3b)^2 = 2(2a)^2 = 8a^2
I think you left out a +6 somewhere, since there are no such terms shown.
Also, if a=3 and b=2, c=72, not 78, so it's a good sign something has been left out.
So, if D is the answer, the question has been garbled. At any rate, you can see how I did it, so maybe you can apply the logic to the correct problem.
Sorry- it was 1/3c = 6b^2 + 2.
I can see your way of substituting, though!