Question

Find the common difference of an ap in which a21-a7=84

Answer 1

let a is the first term and d is the common difference of an AP 
a21 = a + ( 21 -1)d = a + 20d 
a7 = a + (7 -1)d = a + 6d 

now, 
a +20d -a -6d = 84 

14d = 84 

d = 6 ( answer)