Calcium nitrate, Ca(NO₃)₂, is used as a fertiliser…

Step 1. Molar mass. $40\ \text{g/mol} + 2 \times (14\ \text{g/mol} + 3 \times 16\ \text{g/mol}) = 164\ \text{g/mol}$.

Step 2. Nitrogen's part. $2 \times 14\ \text{g/mol} = 28\ \text{g/mol}$.

Step 3. The share. $\dfrac{28\ \text{g/mol}}{164\ \text{g/mol}} \times 100 = 17.1\ \%$.