Answer:
19 inches
Explanation:
You want the middle length of three odd integer lengths whose total is 57.
Middle length
The middle length is the average of the three lengths, so is their total divided by 3:
middle length = (57 in)/3 = 19 in
The middle side length is 19 inches.
__
Additional comment
The perimeter of the triangle is the sum of the lengths of its sides.
If you like, you can write an equation using x for the middle length. Then the other two are x-2 and x+2 and their sum is ...
(x-2) +(x) +(x+2) = 3x = 57
x = 57/3 = 19, as above