Example of ammonia contains 9 grams of hydrogen 42 grams of nitrogen another sample of compound contains 12 grams of hydrogen. calculate the amount of nitrogen in the second sample of ammonia. express you answer with the appropriate units

This is simply a case of ratio and proportion. By the Law of Definite Proportions, the ratio of the masses of hydrogen and nitrogen in any sample of ammonia must be the same.
We know that in a sample of ammonia, for every 9 grams of hydrogen, there  are 42 grams of nitrogen, we can do the following calculation to determine amount of nitrogen in a sample of ammonia that contains 12 grams of hydrogen:

12 grams H x ( 42 grams N / 9 grams H) = 56 grams of nitrogen