6.10 g of boron react completely with in 2.9 g of chlorine to give 29.0 g of the metallic chloride calculate the empirical formula of the chloride

I don't know what but something funny is going on.You are using 6.10 g + 2.9 g = 9.0 g on the left but the product is 29.0 g. Law of Conservation of mass doesn't work that way.