To solve this problem, use dimensional analysis to convert from grams of nitrogen to moles of nitrogen to moles of hydrogen to grams of hydrogen. The conversion factor you will use are the molar masses of N2 and H2 and the mole ratio of H2 to N2, which is the coefficients from the balanced equation.
100 g N2 x (1 mol N2/28.01 grams N2) x (3 moles H2/1 mole N2) x (2.02 grams H2/mole H2) = 21.64g H2
If you know the amount of any reactant or product you can find the amount of any other reactant or produce using this method. Always include the units in your calculations and cancel out those that are in both the numerator and the denominator. If you don't end up with the proper unit, in this case grams, that will alert you to errors in setting up the problem.