Triangle stuff will mark brainliest, etc

4. All angles in a triangle add to 180° and a straight line has an angle range of 180° also. So, 180-63-60 = 57° which is the missing angle inside the triangle. Then to find the complement of the angle that makes up the straight line, take 180-57 = 123°. Also, the Exterior angle theorem states that the exterior angle complement to an interior angle from a parallel line is the sum of the two interior angles that are furthest from this parallel line. So you can apply this and realize the two furthest angles from angle1 is the sum of 60 and 63 which is also 123°.

5. Theorems discussed above are Triangle Angle-sum or the Exterior Angle Theorem.