The chemical reaction equation is already balanced. So, according to this balanced equation, you need 3 moles of Hydrogen gas for every 1 mole of Nitrogen gas. Now we need to find how many moles of Nitrogen is there in 1.4 L of nitrogen. If you remember in the chapter discussing gases and gas laws, there is a conversion factor which is .... At standard conditions (STP), 1 mole of ANY gas would occupy 22.4 L of volume. In other words, if you have 22.4 L of nitrogen, it means you have 1 mole of nitrogen [Remember, mole is just a number of items, but HUGE number => 6.023 x 1023 items in 1 mole]
22.4 L = 1 mole
Therefore 1.4 L of N2 ==> (1 mol N2/22.4 L N2) * (1.4 L N2) = 0.0625 mol of N2
According to the equation, each mole of N2 needs 3 moles of H2
Therefore, 0.0625 mole of N2 would need => (3 mol H2/1 mol N2)*(0.0625 mol N2) => 0.1875 mol of H2
Now you need to convert the moles of H2 to Liters of H2 by using the same conversion factor.
==> (22.4 L/1 mol)*(0.1875 mol) ==> 4.2 L of H2