1. Identify the atomic masses of the elements:

* Carbon (C): 12.011 amu

* Sulfur (S): 32.065 amu

2. Multiply the atomic mass of each element by its subscript in the formula:

* Carbon (C): 12.011 amu/atom * 3 atoms = 36.033 amu

* Sulfur (S): 32.065 amu/atom * 2 atoms = 64.130 amu

3. Add the results from step 2 to find the formula mass:

* Formula mass of C3S2 = 36.033 amu + 64.130 amu = 100.163 amu

Therefore, the formula mass of C3S2 is 100.163 amu.