Answer : Third side will be x-3y+8 .
Explanation : 
We have given that, 
Length of first side of triangle = 5x+y
Length of second side of triangle = 7+x
Perimeter of triangle = 7x-2y+15
 Now , we know that " Perimeter means sum of boundary", 
So, 

So, Length of third side will be x-3y+8.