Answer:
5 and 7 (if the similar rectangle is smaller)
Explanation
What makes shapes similar are their proportions so if the numbers of the sides are proportional, so are the sides themselves.
For example, 2/4 is proportional to 1/2 because 2/4 reduced equals 1/2.
This method works for the side of the rectangle. If you divide 10 and 14 by 2, you get the numbers 5 and 7.
5 and 7 are only one of the several answers that could be used: 20 and 24, 30 and 42, etc. Those are the answers if you're looking for a bigger rectangle that is also similar.