Linear pair: Two adjacent angles form a linear pair if their non-shared rays form a straight angle (a straight line)

1 and 2 form a linear pair because they are

adjacent angles with the common HI and

their two non-shared rays GH and HJ form a straight angle

Linear Pair Property

The angles that form a linear pair are supplementary

If 1 and 2 form a linear pair, then m 1 + m 2 = 180⁰