Please Explain, thanks

The present value (at n=0) of the following cash flow stream is $6000 when discounted at 12% annually. That stream consists of payments made at the end of each of four periods. One of the payments, the second one, is missing. The others are $1,000 at n=1, $2,000 at n=3, and $2,000 at n=4. What is the value of the missing cash flow