To find molar mass, multiply the molar mass of each atom by the number of that type of atom and then add the masses. I have done it in detail here showing units.
The molar mass is the mass in grams of 1 mol of a substance. #H_2CO_3# has 2 atoms of H, 1 atom of C, and 3 atoms of O. So 1 mole of it has 2 mol H, 1 mol C, and 3 mol O. The mass of one mole of an atom is the molar mass in #g/(mol)# on the periodic table. So the molar mass is:
#2# #mol# #H* (1.01\ g\ H)/(mol\ H_2CO_3)= (2.02\ g\ H)/(mol\ H_2CO_3)#
#1# #mol# #C* (12.01\ g\ C)/(mol\ H_2CO_3)= (12.01\ g\ C)/(mol\ H_2CO_3)#
#3# #mol# #O* (16.00\ g\ O)/(mol\ H_2CO_3)= (48.00\ g\ O)/(mol\ H_2CO_3)#
#(2.02\ g\ H)/(mol\ H_2CO_3)+(12.01\ g\ C)/(mol\ H_2CO_3)+(48.00\ g\ O)/(mol\ H_2CO_3)=(62.03\ g\ H_2CO_3)/(mol\ H_2CO_3)#
