Formula mass unit is the sum of all the atomic weights that make up the substance.
In the case of
Na = 1
H = 1
C = 1
O = 3
Next is to get the atomic weights (just take a look at the periodic table).
Atomic weight of Na = 22.989769 amu (atomic mass unit)
Atomic weight of H = 1.00794 amu
Atomic weight of C = 12.0107 amu
Atomic weight of O = 15.9994 amu
Then add the atomic weights according to the formula.
22.989769 amu + 1.00794 amu + 12.0107 amu + (3 x 15.9994 amu)
= 84.0 amu