With the number being x, we have 3x^2>6(x/9). Expanding, we get 3x^2>6x/9 and 3x^2>2x/3. Dividing by x, we get 3x>2/3 and dividing by 3 we get x>2/9
So, Using your conversion chart(if you have one) you'd see 1 liter makes 1000 milliliters. So, if you have half of a liter(0.5), then you have 500 milliliters. If he adds 300 more, then you get 800 milliliters total, which is also equal to 0.8 liters. Hope this helps!
