Mass of calcium nitrate: 1200 kg
The idea behind this problem is that you first need to determine how much nitrogen is supplied per mole of calcium nitrate, then use this value to determine how much you'd get per gram of calcium nitrate.
In other words, you need to determine the percent composition of nitrogen in
This important thing to notice is that you get 2 moles of nitrogen per mole of calcium nitrate, which means that you have
This means that, for every 100g of calcium nitrate, you get 17.1 g of nitrogen.
Since you're dealing with a 10-hectare field, you need to supply a total of
You can express this value in grams to get
Since you need 100g of calcium nitrate to get 17.1 g of nitrogen, you can determine that
You can convert this back to kilograms if you want
Now, I'll leave the answer rounded to two sig figs, despite the fact that you only gave one sig fig for the mass of nitrogen and area of the field.