Question 5401
Look at it this way:
If x = the initial investment, then the percentage increase * the initial investment = final value
If it is doubling each 7 year period, then there are 4 periods of 7 years in our time frame of 28 years
This is represented as {{{2^4x}}}

Another way to look at it, is as follows: 
2x= the investment in year seven, 
2(2x)= 4x the investment in year 14 
2(4x)= 8x the investment in year 21 
2(8x) = 16x the investment in year 28 
So if we plug in the value of x = ($1000), then 

16($1000)=$16,000