Ok, I'll give you hints that will help you get to the answer...
1. Write down the chemical formula of Mercury(II) nitrate.
2. How many moles of mercury can be precipitated from one mole of mercury(II) nitrate? (hint : use the chemical formula of solid mercury and that of mercury(II) nitrate)
3. 1L of outfall sample contains 134 µg mercury nitrate. So, how much of it is present in 16kL?
4. Final clue : convert mass of mercury nitrate to moles of mercury nitrate. Using hint 2, find moles of mercury. Convert moles of mercury to mass of mercury
Hope this helps