Answer:
5x +4y
Step-by-step explanation:
The perimeter of a triangle is the sum of side lengths. That fact can be used to find z, the length of the third side.
(2x +3y) +(5x -2y) +z = 12x +5y . . . . sum of sides is perimeter
z = (12x +5y) -(7x +y) . . . . . . . . . . . . subtract (7x+y) from both sides
z = 5x +4y . . . . . . . . . collect terms; the length of the third side
The third side of the triangle is (5x +4y).
