I think u meant "9.03g of Magnesium combined completely with 3.48g of Nitrogen to form a compound. what is the percentage composition of this compond?"
We work by percentage composition by mass.
Using Law of Mass Combination, and that we know
9.03g of Mg reacts completely with 3.48g N2,
the mass of the substance formed = 9.03 + 3.48 = 12.51g
percentage composition of Mg would be
= 9.03g / 12.51g X 100%
U can obtain the percentage composition of mass for nitrogen using the same method. The idea behind is that u need to know: 100g of the substance formed would contain how many grams of each elemental constituent.