He starts with 6 liters of an 8% boric acid solution.

8% of 6 is 0.48, so those 6 liters of liquid contains just 0.48 liters of pure
boric acid and the rest is water.

Now he adds x liters of water and now he has 6+x liters of liquid.

That 6+x liters of liquid still contains just 0.48 liters of pure boric acid.

That 0.48 liters of pure boric acid is now only 5% of those 6+x liters of liquid.

So 0.48 = 5% of 6+x
   0.48 = 0.05(6+x)
     48 = 5(6+x)
     48 = 30+5x
     18 = 5x
    3.6 = x

Answer: He needs to add 3.6 liters of water.

Edwin