how do i find out the mass of mercury in precipitation?

If the lake area is 2.52 km2(squared), and the concentration of Hg in precipitation is 6.7 ng/L, and the amount of precipitation falling on the lake is 202 mm/year, what is the mass of mercury in the lake during that year period?
If you could show me HOW to do this question and the steps involved that would be great!
 
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