Question

a21-a7=84
ARITHMETIC PROGESSION
PLZ AAJ PAPER HAI​

Answer 1

I think that still the quest. is incomplete...

Answer 2

a21 - a7 = 84

So the AP :- a21 - a7 = 84

Formula is used :- an = a +(n-1)×d

as we can write a21 = a + 20d

a7 = a+6d

substituting value in the first

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

bracket open and sign change

a + 20d - a - 6d = 84

+ 20d - 6d = 84

14d = 84

$d \: \: = \frac{84}{14} = 6$