its simple :
you just have to multiple the number which is in front of it to its atomic weight.
in this case it is like this :
=1 × atomic no. of Na + 1 × atomic no. of O + 1 × atomic no. of H
=1 × 23 + 1 × 16 + 1 × 1
=23 + 16 + 1
=40
one more :
Molecular weight of Na₂CO₃
= (2 x Atomic weight of Na) + Atomic weight of C + (3 x Atomic weight of O)
= (2 x 23) + 12 + (3 x 16)
= 106
