The unbalanced equation is BF3 + Li2 SO3 --> B2 (SO3)3 + LiF, where the numbers to the right of the symbols are subscripts. The balanced equation is 2 BF3 + 3 Li2SO3 ---> B2 (SO3)3 + 6LiF. The numbers in front of every formula (compound) are the respective coefficients. You can check that the equation is balanced counting every element: There are 2B in the side of the reactants and 2B in the side of the products; there are 6F in the side of the reactants and 6F in the side of the products; there are 6 Li in the side of the reactants and 6 Li in the side of the products; and there are 3 SO3 in the side of the reactants and 3 SO3 in the side of the products. So, the coefficient of the LiF is 6.