Let, t be the time it took for Karen to catch up with Renee;
Karen: d= 28t
Renee: d= 20(2 +t). Renee has already travelled for two hours.
The distance they travelled will be equal, when Karen catches up with Renee, so set the equations equal to each othe,
28t = 20(2 + t)
28t = 40 + 20t
8t = 40
t = 5
The answer is after 5 hours.