2Li+2H2O=2LiOH+H2
We need to make number of the atoms of each reactant(the elements from the left side)equaled to the products(the elements from the right side)
Li is 2 on the left side so in order to make it balanced we add a 2 in front of LiOH.
For the H2O we take a look at the right side there are 2 atoms from H2 and 2 atoms of H for 2LiOH therefore we have 4 atoms of H in the right side and only 2 in the left side.So,to make them equal we will add a 2 in front of H2O.
Hope,I explained well.