To calculate the %N in (NH4)2C2O4 the first step is to count how many of each atom is in the molecule. In this case you have in the parenthesis 2N and 8H then outside of the parenthesis you have 2C and 4O. To figure out the % N in the molecule you have to add up all of the atomic masses of each individual atom together. Then you can divide the atomic mass of 2N by the overall atomic mass of the entire molecule. That number you multiply by 100%. That is how you get your answer. I also am showing the work below in case that would help you as well.

Using the atomic masses of:

15.999g for Oxygen (O); 14.007g for Nitrogen (N); 1.008g for Hydrogen (H); 12.011g for Carbon (C)

to figure out the overall atomic mass you would do:

(4 x 15.999) + (14.007 x 2) + (1.008 x 8) + (12.011 x 2) = 124.096g

the atomic mass for Nitrogen is:

14.007 x 2 = 28.014g

Then you divide the mass of nitrogen by the overall mass: