I am stuck on a geometry lesson. It is on the Angle Sum Theorem. It said we have an obtuse trisngle with angles 1, 2, and 3. Then they extend the line out as a dashed line top and bottom parallel to each other. It then he says we have angle 1 value would also be true on the opposite side of the triangle like as a transeversal intersected 2 parallel lines making those angles alternate interior angles. He does the sme with 2. then says 1 plus 2 plus 3 equal 180 and says it twice. What does that mean?​