Answer:
or

.
Explanation:
Suppose, the first side is a, the second side is b, and the third side is c. So:
→ a = s - 4
→ b = s
→ c = (s - 4) + 3
→ a + b + c = 15
First step, find the value of s
a + b + c = 15
(s - 4) + s + (s - 4) + 3 = 15
3s - 5 = 15
3s = 15 + 5
3s = 20

So, the third side is
c = (s - 3) + 4
c = s + 1
c =


.
Happy to help :)