At the end of 2016, Massive Dynamic reports that the book value of its stockholders' equity is $6 billion. In 2017, the company reports the following transactions:

• Launches first share repurchase program and buys back 5 million shares during the year at $400 per share.
• Issues 40 million shares of preferred stock during the year at $100 per share.
• Issues 4 million shares of class B common stock during the year at $400 per share.
• Grants 3 million restricted stock at the beginning of the year with a 3-year service/vesting period at $400 per share.
• Completes 2 for 1 stock split at the end of the year.

What is the stockholders' equity balance at year-end 2017?