(NH4)3PO4
For this, we need the atomic masses for the next elements:
N = 14
H = 1
P = 31
O = 16
To get the gram-formula mass of this molecule, we must add every atomic mass and multiplying by the correct number of atoms that appear in the molecule.
For (NH4)3PO4 = 3 x 14 (N) + 3 x 4 x 1 (H) + 1 x 31 (P) + 4 x 16 (O) = 149 g/mol
Answer: 149 g/mol