In this case Year = 1960 + x , so the year 1970 means x =10 and 1990 gives x = 30.
What the equation gives is the number of cubic gas used in a year (not the year itself), so if you feed in the numbers 10 to 30 as x, and see what values you get it will tell you which year's consumption is closest to 25,700,000,000,000.