Answer: a. 17 years
Explanation:
The annual fee is $750.
The number of years that it will take for the lifetime subscription to be cheaper than the annual fee is the number of years it will take the the annual fee of $750 paid every year to have a present value of $7,000 given an annual interest rate of 8.9%.
Using the Excel function of NPER this is;
=NPER(rate,pmt,pv,fv,1)
= NPER(8.9%,-750,7000,0,1)
= 16.88 years
= 17 years