It means that 


     +  =       (1)    (at n= 1)

     +  =      (2)    (at n= 2)

or

     +  =        (1')  

     +  =       (2') 


Now your task is to solve the system (1'), (2') for unknown "a" and "b".

First step, divide by 2 all the terms in equation (1');  keep equation (2') as is.


     +  =        (3)  

     +  =        (4) 


Next step subtract equation (4) from equation (3).  You will get then


                      = ,   or


                      =  = 

which implies  b = -2.


Then from equation (1) you can find "a".


The rest is just arithmetic, which I leave to you to complete on your own.