There are two different compounds of sulfur and fluorine.
in sf6, the mass of fluorine per gram of sulfur is 3.55 g f/g s.
in the other compound, sfx, the mass of fluorine per gram of sulfur is 1.18 g f/g s.
what is the value of x for the second compound?
express your answer as an integer.

1.18 x 3 = 3.55 
find  ratio of F to F in each compound
.   according to law of multiple proportions  that the masses of one element which combine with a fixed mass of the second element are in a ratio of whole numbers.
now  F is  "one element" and S has  "fixed mass",
the ratio of F6 to Fx = 3:1 
thats why  x= 2
there is less F in SFx
  the ratio is 3:1.
 dividing 6 by 3 and you get 2

The correct answer should be a.

Sorry im just trying to get answers like you are