Question 1078560
Let W:Water on the initial solution
    A:Acid on the initial solutiom.
    X:Water need to be added.

1) A+W=6
2) A/(A+W)=30%
3) A/(A+W+X)=12%

Using 1) and 2) 

A/6=30%, solving for A, A=1,8 liters.

So in 3) 1,8/(6+x)=12%, solving for x, 6+x=15, x=9 liters.

Answer: you must add 9 liters of water in order to make the solution of 12%.

@natolino_