Answer:
x - 3y + 8
Explanation:
The perimeter of a triangle is sum of all the three sides.
One side is 5x + y
Another side is 7 + x
Let third side be m
And perimeter is 7x - 2y + 15
We simply sum the 3 sides and equate it to expression of perimeter and find m(third side). Shown below:
5x + y + 7 + x + m = 7x - 2y + 15
6x + y + 7 + m = 7x - 2y + 15
m = 7x - 2y + 15 - 6x - y - 7
m = x - 3y + 8
This is the length of the third side "x - 3y + 8"