Perimeter = Sum of three sides.
Let the third side be = t
Perimeter = 8x + 3y
8x + 3y = (4x + y) + (2x - 3y) + t
8x + 3y = 4x + y + 2x - 3y + t
8x + 3y = 4x +2x + y - 3y + t
8x + 3y = 6x - 2y + t
8x - 6x + 3y + 2y = t
2x + 5y = t.
Therefore third side = 2x + 5y