How do you find c in quadratic sequences

Ok so I know the formula for any quadritc is a(n^2)+bn+c

And I know how to find a (half the 2nd difference)

And I know how to find b (second difference after taking a(n^2) away from nth term)

But how do I find c?

Put your values for a and b into the equation for n e.g. if a = 3 and b = 2 you get

3n^2 + 2n + c (not the example you gave by the way)

Now look at a value for n e.g if n = 3 and the value is 17 then you get

3.(9) + 2.3 + c = 17 so in this case c = -16

Yes - it really is that simple.