Supplementary angles are two angles whose measures add up to 180° . The two angles of a linear pair , like ∠1 and ∠2 in the figure below, are always supplementary. But, two angles need not be adjacent to be supplementary. In the next figure, ∠3 and ∠4 are supplementary, because their measures add to 180° so
angle ∠ABC 90° therefore subtract the 90 to 180 to get the final answer
180-90=90
Final Answer is ∠QRS 90
