CREATE TABLE table_79744 ("Name" TEXT, "Birth year and age" TEXT, "Net worth US$  ( billions ) " FLOAT, "Citizenship" TEXT, "Source" TEXT)

What's the source of wealth of the person worth $17 billion?
SELECT "Source" FROM table_79744 WHERE "Net worth US$ (billions)" = '17'
SELECT "Source" FROM "table_79744" WHERE "Net worth US$ (billions)" = '17'