To determine the molar mass of a compound we need to find every molar mass of each atom in the formula
1) The molar mass of every atom
Lead (Pb) = 207,2 g
Nitrogen (N) = 14,0067 g
Oxygen (O) = 15,9994 g
2) How many times does an atom appear?
Lead: there is just one Lead atom
Nitrogen: there are two Nitrogen atoms
Oxygen: there are six oxygen atoms
The molar mass of Pb(NO3)2 is equal to
(207,2*1)+(14,0067*2)+(15,9994*6) = 331,2098 g