How do you find the formula mass of ammonium carbonate (NH4)_2CO_3 ?

Break up the compound first, just so you know what atoms are present.
In that case, N, H, C, and O are atoms present.

Next step, refer to your periodic table to find out the formula mass of each atom.
N = 14.0 g/mol
H = 1.0 g/mol
C = 12.0 g/mol
O = 16.0 g / mol

Now look back at the formula again to see the total number of atoms.
Inside the bracket, there is no number after the N, that means it has one atom, however, it is surrounded by the bracket with the 2 outside of it, meaning there are 1xx2 = 2 atoms of Nitrogen.
Follow the same structure to find the number of atoms in Hydrogen - in that case 4xx2 = 8 atoms of Hydrogen.

Now find the number of atoms for C and O. Since there are no brackets that interfere CO_3, you can simply tell that there are 1 atom of Carbon and 3 atoms of Oxygen.

Now you can find the formula mass of (NH)_4 CO_3.
Multiply the formula mass of each individual atom by the total number of atoms present.
N = 14.0 g/mol xx 2 = 28.0 g / mol
H = 1.0 g/mol xx 8 = 8.0 g / mol
C = 12.0 g/mol xx 1 = 12.0 g / mol
O = 16.0 g / mol xx 3 = 48.0 g / mol

Add up the formula mass together, therefore, it will give you the overall formula mass of (NH)_4CO_3.
28.0 + 8.0 + 12.0 + 48.0 = 96.0 g/mol is the formula mass of (NH)_4CO_3.