You are presented with an investment opportunity that will give you the following stream of cash flows: nothing for the next $3$ years; starting at the following year, an amount of $$4,000$ per year until year $12$; and after that year, then an amount of $$8,000$ per year until year $21.$ If your required rate of return $($APR$)$ is $11\%$ compounded annually, what is the future value at end of year $21$ of these cash flows?