She will run and swim again on the same day after 21 days.
Step-by-step explanation
Juanita runs every third day and swims every seventh day.
For finding the number of days she will run and swim again on the same day, we just need to find the LCM(Least common multiple) of 3 and 7.
As there is no common factor between 3 and 7, so for finding the LCM, we will just multiply 3 and 7.
So,

Thus, she will run and swim again on the same day after 21 days.