You know that the three angles of a triangle add up to 180°. If one angle is 38° and another angle is 67°, then the third angle must be 180°-38°-67° = 75°.
You have a straight line across the bottom, and three angles meeting in the middle of it. Those three angles add up to 180°. One of them is 75°, another is 50°, so the third angle must be 180°-75°-50° = 55°.
That leaves you with one unknown angle. ? = 180°-40°-55° = 85°
