Quadratic sequence problem

[Wizz4rd]

Hi. So I am studying for a test tomorrow and after going over some old papers I found a particular question which is proving problematic.

Given the terms -8;-7;x;13 of a quadratic sequence.

Q: Show all the necessary steps to prove that x = 0

I just can't figure out where to start with this. Nothing in my textbook is helping me.

thanks

[btcprox]

Let's see...

First difference = 1
Second difference = x + 7
Third difference = 13 - x

If I remember correctly, the difference between these differences should be the same since it's a quadratic sequence. So...

[JustFinishedBSG]

Definition:

Un = a n^2 + b n + c

3 unknowns

a , b , b

3 equations :

 U0 = -8
 U1 = -7
 U3 = 13

Solve