Can you explain the process of finding the NPV for the following question (including formula, etc):

A proposed nuclear plant will cost 2.2 billion to build and then produce cash flows of 300 million a year for 15 years. After that period (in 15 years), it must be decomissioned at 900 million. What is the NPV if the discount rate is at 6% ?

(can you also explain what decomissioned is)

thanks!