→2 Na + Cl₂ -----------> 2 NaCl

In the above equation, Chlorine exists as a diatomic gas, and thus its subscript 2 cannot be altered. Instead you'll have to balance, only by PLACING suitable coefficients to each participant in the above reaction.

Since there are 2 chlorine atoms on the reactants side, You MUST balance the number of chlorine atoms on either side by placing 2 in front of NaCl .

But then, we have 2 Sodium(Na) atoms on the products side which too can be balanced by placing 2 in front of Na on the reactants side.

→2 H₂ + O₂ -----------> 2 H₂O

If you get the first ONE, then there cannot be a problem balancing this one.

→N₂ + 3 H₂ -----------> 2 NH₃

There are 2 Nitrogen atoms on the reactants side, which means that we have to place a '2' in front of NH₃ in the product side in ORDER to balance the number of nitrogens;

Now as there are 6 hydrogen atoms on the products side, Place a '3' in front of H₂, so that there are 6 Hydrogen atoms on the reactant side too)

→N₂ + O₂ -------------> 2 NO

This one's the easiest as Balancing any one of nitrogen or OXYGEN will balance the whole equation.

→HCl + NaOH ----------> NaCl + H₂O

This is a simple acid-base reaction (also called neutralisation reaction), the reaction doesn't need to be balanced as it already follows the law of conservation of mass.

Hope it helps!!