Question

If the common difference of an ap is 3 then find the value of a 17-a12

Answer 1

if 17-a12=0
a=12-17
a= -5

Answer 2

$a_{17} -a_{12} =a+16d-$$(a+11d)$

=a+16d-a-11d

=5d

=5*3

=15