getting the "n" in quadratic regression

How do you get the n manually from the quadratic regression formula:

y=ax^2+b+c

(Σx^2)a + (Σx)b + n c = Σy

(Σx^3)a + (Σx^2)b + (Σx)c = Σxy

(Σx^4)a + (Σx^3)b + (Σx^2)c = Σx^2y

or is n given? if it is, where does it come from?

[Oleg Komarov]

The summation for i = 1 to N of a constant c, is N*c. This is a basilar summation property.