here is the que

Using (a²– b²= (a+b)(a-b)
2016²- 2013²= 3( 4029)
2015² –2012²= 3(4027)

Similarly u can proceed and take out 3 common from all the pairs.
So it will be divisble by 3. The other term in the bracket will be AP. You can solve and proceed.

Approved