In 1996 a total of 14,968,000 taxpayers in the U. S. filed their individual tax returns electronically. By the year 2004 the number increased to 66,290,000. What is the geometric mean annual increase for the period?

So there's 8 years of increase. You start with 14,968,000 and end with 66,290,000.

The compounded increase per year would be:

14,968,000 * (1 + rate increase per year)^8 = 66,290,000

Then rearrange and solve for the rate of increase per year.