A pair of parallel lines is cut by a transversal. At the first (upper) intersection, the interior angle below the line, on the right of the transversal, is 42 degrees. At the second (lower) intersection, angle a is the interior angle, between the two parallel lines, on the same right-hand side of the transversal (the co-interior / allied angle to the 42-degree angle). Find a and give a short reason.
Figure (to be drawn): Two parallel lines with transversal. At the first (upper) intersection, the angle marked 42 degrees is below the line, on the right of the transversal, i.e. between the two parallel lines (interior). At the second (lower) intersection, angle a is below the transversal's crossing but above the lower line, i.e. between the two parallel lines, on the same (right) side of the transversal as the 42-degree angle - the co-interior angle to it.
(3)