A straight line has an angle of 180 degrees. We are assuming this is a straight line, meaning angle 1 and angle 2 together should add up 180 degrees. If the measure of angle 2 is 102, the measure of angle 1 should be supplementary, which is another mathematical way of saying it adds up to 180 degrees. The measure of angle 1 has to be 78 degrees to make the angles add up to 180 degrees.
