you for asking question, look at the attached image to understand question better.
if you look at the yellow square you will see it is a square of 90°.
together with angles 1 and 2, form a straight angle so we have:
subtracting 90° from both sides we have:
if the sum of the measures of 2 angles is 90°, the angles are called complementary.
the sum of the measures of angles 1 and 2 is 90°,
then these 2 angles are complementary.
so, are complementary
if there is any confusion leave a comment below.