arre yaar...a²-b²=(a+b)(a-b) so (a+b) and (a-b) are two factors.....for a prime number the factor shud be either itself or 1

we cannot have (a+b)=1 as a and b are positive integers...

so we can have a-b=1

multiplying both sides by (a+b) we have (a²-b²) =(a+b) 

hence the problem....

and btw vatika i guess u got a so called "AIR 21"...na.....so go ahead and tell us wat branch u got....cmon'...