Answer:
See below.
Explanation:
Start with triangle ABC with side BC lying on line n.
Draw a line through point A parallel to line n, and call it line m.
Angles 1 and 3 are alternate interior angle of parallel lines and are congruent.
Angle 2 and 4 are alternate interior angles of parallel lines and are congruent.
The sum of the measures of angles angles 1, BAC, and 2 is 180 since they lie on a line.
m<1 + m<BAC + m<2 = 180
Now we substitute in the angles we know are congruent by alternate interior angles (written above).
m<3 + m<BAC + m<4 = 180