Grouper Gas, an oil and gas company, had the following information on its financial statements for the fiscal years ended December 31. All figures are in millions of US dollars.

		2021			2020			2019			2018
Cash provided from operating activities		$1,050			$496			$158			$455
Cash used in investing activities		(3,375)			(561)			(739)			(212)
Cash provided from financing activities		1,196			1,467			393			425
Net change in cash		(1,129)			1,402			(188)			668

(a)

Calculate Groupers free cash flow for 2018 through 2021. (Enter amounts in millions. Enter negative numbers using either a negative sign preceding the number, e.g. -45 or parenthesis, e.g. (45).)

		2021		2020		2019		2018
Free cash flow		$		$		$		$