How to solve plz , i have an exam tommorow

 Was .08 off its 4.08 like that guy explained 

First change the grams to moles using the molar mass of the compound which can be calculated by adding the mass of each atom from the periodic table.

the use mole-mole ration from the given reaction equation. then convert the moles to mass back again using the molar mass of that compound.

molar mass NaF= 42.0 g/ mol
molar mass MgF2= 62.3 g/ mol

moles NaF= 5.5 / 42.0 g= 0.131 mol NaF

2 moles NaF = 1 mole MgF2

so 0.131 mol NaF/ 2= 0.0655 mole MgF2

grams MgF2= 0.0655 moles x 62.3= 4.08 grams

or simplified

5.5 g NaF (1 mol NaF/ 42.0 g) (1 mol MgF2/ 2 mol NaF) (62.3 g/ 1 mol MgF2)= 4.08 grams