Answer: about 10 years.
1) annual rate of growth = 6.97 % => growth factor = (1 + 0.0697)^n, where n is the number of years.
2) doubling time growth => factor = 2
3) equation: 2 = (1 + 0.0697)^n
4) solve the equation
- take logarithms: lg(2) = n * log (1+0.0697)
- divide both sides by log(1+0.0697): n = log(2) / log (1.0697)
- result: n = 10.29
Conclusion: the double time is about 10 years.