Answer:
a*2=b
Or
a=0, b=0
Explanation:
if a=0 and b=0, then 0=0, it'd be correct, but I'm sure that's not the answer they intend you to pick. You should still put that as one of the answers, however.
-3a+6b=a+4b
+3a +3a
6b=4a+4b
-6b -6b
0=4a-2b
/2 /2 /2
0=2a-b
so, we now know that b is twice the amount that a is, and actually, that's the solution for the entire thing.
So, the answer is a*2=b, because the equation works for any set of numbers, as long as it follows this rule.