The relative formula mass of a compound is simple to calculate once you know how.
It is very similar to the atomic mass of an element, except instead of only being the mass of that one element, it is the mass of the whole compound.
So how do you work it out? Simple you add up the atomic masses of all the elements in the compound.
The relative atomic masses of these elements are: H=1; C=12; O=16; Na=23
(Note: These will be on the periodic table you are given, I have just put them here for ease)
Work out the relative formula mass of NaHCO3
So the calculation is...
(123)+(11)+(112)+(316) = 84g/mol