The chemical equation N2 + 3H2 ---> 2NH3 tells us that in order to make two molecules of NH3, we need one molecule of N2 and three molecules of H2.
To figure out how many moles (which is just a way of measuring how much of a substance you have) of H2 we need to react with 14.0 g of N2, we can use the information from the equation.
First, we convert the 14.0 g of N2 to moles (which means we're figuring out how many pieces of N2 we have, because 1 mole = Avogadro's number of particles, or roughly 6.022 x 10^23).
14.0 g N2 x (1 mol N2/28 g N2) = 0.5 mol N2
Then, we use the mole ratio from the equation to figure out how many moles of H2 we need:
0.5 mol N2 x (3 mol H2/1 mol N2) = 1.5 mol H2
So we'd need 1.5 moles of H2 to react completely with 14.0 g of N2.