math again Watch
Then you look at the difference between those. That's constant at 2. Call that the second difference
Basically as a rule (it'll do for GCSE), when the second difference is constant, it's a quadratic series - i.e. there's an n2 somewhere in the nth term. If it's 2, then it's n2, if it's 4 then it's 2n2, etc.
Once you've got that down, the rest should be easier to work out.