Consider this reaction:

2B (s) + 3F2 (g) 2BF3 (g).

Which equilibrium expression is correct?

Le Chatelier's principle states that increases in molar concentrations of products or reactants changes the equilibrium of the equation. By the given equation, assuming solids are not included in the determination of equilibrium, an increase in reactant 3F2 (g) also increases the product 2BF3 (g). Should solids be included, 2B (s) will decrease for the product to increase.