At the end of your first year of employment after graduation, you received a $$12,000$ bonus. You invest $$4,000$ in a retirement account and spend the rest. Five years later, your investment has grown to $$5,400.$ If$,$ instead, you had invested the entire $$12,000$ bonus in the retirement account and the investment increased by the same proportion, how much would your investment be worth at the end of the same $5-$year period?