The answer is 15, but I don't know how to get to that answer.
For this question it's useful to define the unknown. Let's call x the number of liters of 12% acid solution to be used.
Since we're going to mix the solutions, our new solution will have (x+5) liters.
How much acid is present in the new solution? Well, the original 5 liters contribute 20%, that is
20*5/100 = 1 liter of acid
The added solution contributes 12%, that is
12*x/100 = 0.12*x liters
So we have a total of 1 + 0.12*x liters of acid in the (x+5) liters of solution (see above) and we want the acid to be 14% so:
1 + 0.12*x = 14*(x+5)/100
and now we just need to solve for x:
1 + 0.12*x = 0.14*(x+5)
1 + 0.12*x = 0.14*x + 0.7
1 - 0.7 = 0.14*x - 0.12*x
0.3 = 0.02*x
30 = 2*x
15 = x
And that's the correct answer!