Could someone help me? i'd appreciate it thank you

Given: x + y = 6. if xe (-15, -6, -1), then which of the following sets of ordered pairs are solutions? © {(-15, -21), (-6, -12), (-1, -7)} [(-15, 21), (-6, , 7)} {(-15, 21), (-6, 12), (-1, -7)}

What is the missing reason in step 5? linear pair postulategivendefinition of complementary anglescongruent complements theorem

What is the range? can’t seem to find it out