Richay...try to identify the range of numbers that the solution definitely *isn't*.
For example...say I let n+2 = 4...what is the product of 4 and -6?...what is the product of 2 and 3? ( since n+2 = 4, our third number...our first and second are n and n+1 )...
secondly can n be somewhere around +/- 10 million?...whereabouts does n lie?
Be aware that potentially the problem might be expressed in any of these ways:
-6n = (n+1)(n+2)
-6n = (n-1)(n-2)
-6n(n-2) = (n-1)(n)
-6n(n+2) = (n+1)(n)
Only two of these is any help, the other two will not...which two cannot help and why?
(By finding the range of numbers that your solution isnt, you should be able to solve it just by inspection and then you should try to justify your choices by choosing and solving the correct quadratics)