If the sum of 8th and 9th of an AP is 72 and 4th term is -6,find the common difference

Answer 1

Using your data in the definitions of AP ...
a+7d + a+8d = 72
2a + 15d = 72

a + 3d = -6
a = -6 - 15d

sub that into the first equation:
2(-6-15d) + 15d = 72
-12 -30d + 15d = 72
-15d = 84
d = -84/15

Answer 2

Thanks i want further explanation