Explanation:
Triangle side length rule says the sum of any two sides must be greater than the third side....let 'x' be the missing side, then:
3.7 + x > 5.2 so x > 1.5
3.7 + 5.2 > x x < 8.9
5.2 + x > 3.7 true already
so without any constraints:
1.5< x < 8.9 BUT there is a constraint....the perimeter is <= 15
so x + 8.9 <= 15
shows x <=6.1
then the range becomes a bit restricted to:
1.5 < x <= 6.1 ( the other constraint is already met as
8.9 + 1.5 is greater than 10 )