7. Find the accumulated value at the end of 12 years of a fund where $1,000 is deposited at the beginning of each year for the first 4 years, $2,000 is deposited at the beginning of each year for the subsequent 4 years, and $3,000 is deposited at the beginning of each year for the final 4 years. Express your answer

   in terms of a sum of accumulated values s