y = 3a + 2b. The measure of the third side is 3a + 2b
The easiest way to solve this problem is using the perimeter of a triangle equation P = x + y + z, where x, y, and z are the sides of the triangle.
With P = 9a + 2b, x = a - b, and z = 5a + b, we can calculate the missing third side y as follow:
Clear y from the perimeter of a triangle equation
y = P - (x + z)
Solving
y = 9a + 2b - (a - b + 5a + b)
y = 9a + 2b - a + b - 5a - b
Grouping the equal terms
y = 9a - a - 5a + 2b + b - b
y = 9a - 6a + 3b - b
y = 3a + 2b