Question

solve this the common differences of an AP

Answer 1

hope it may help you

Answer 2

Given,

a21 - a7 = 84

We can write ,

a21 = a + ( 21-1 )d

a21 = a + 20d

Similarly,

a7 = a + 6d

Substitute the above values in given

a + 20d - (a + 6d) = 84

a + 20d -a - 6d = 84

14d = 84

d = 84/14

d = 6

Glad to help you.