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,
[tex]Perimeter = 5x+y+7+x+\text{ third side}\\7x-2y+15=5x+y+7+x+\text{third side}\\7x-2y+15-5x-y-7-x=\text{third side}\\x-3y+8 = \text{third side}[/tex]
So, Length of third side will be x-3y+8.
