f(x) = a·x^3 + b·x^2 + c·x + d
f(0) = 0 --> d = 0
f'(0) = k --> c = k
f(3) = 0 --> 27·a + 9·b + 3·c + d = 0
f'(3) = 0 --> 27·a + 6·b + c = 0
Löse das Gleichungssystem und erhalte: a = 1/9·k ∧ b = -2/3·k ∧ c = k ∧ d = 0
f(x) = 1/9·k·x^3 - 2/3·k·x^2 + k·x
