Question

what is the cubic function for points (2,0) , (4,0), and (1,0)

Answer 1

Any cubic with those zeros is f(x) = a(x-1)(x-2)(x-4) for some nonzero constant a.

If you want the monic cubic (leading coefficient 1):
f(x) = (x-1)(x-2)(x-4) = x^3 - 7x^2 + 14x - 8.