Novacain which contains carbon, hydrogen, nitrogen, and oxygen is 12% nitrogen. If novacain contains two nitrogen atoms per molecule, what is the molecular weight of novocain?

Did you try Google?

12% nitrogen means that 12% of its mass is nitrogen.

Since you know the atomic mass of a nitrogen atom, you should know how much mass the two atoms of nitrogen have together.

Then, use proportions:
12% -> ? g
100% ->?? g

That would be the molecular weight of novacain or novocain whatever it's called.