You need to determine the number of atoms of nitrogen present in 7.50 g of the compounds you have provided.
I'm going to explain how you can find this for one of the compounds. You can use the same method to find the number of atoms of nitrogen in the others.
One mole of any compound contains 6.0221×10^23 (which is also called the Avogadro Constant) molecules.
To find the number of atoms of nitrogen, we first need to find how many moles of the compound are present in 7.5 g. This is done by dividing 7.5 by the molar mass of the compound.
The molar mass of (NH2)2CO is (14+2)*2+12+16 = 60 g. In 7.5 g we have by 7.5/60 = 0.125 moles. There are 0.125*6.0221×10^23 molecules of (NH2)2CO in 7.5 g. Each molecule has 2 atoms of nitrogen. Therefore the number of atoms of nitrogen in 7.5 g of (NH2)2CO is 2*0.125*6.0221×10^23 = 1.505*10^23.