f(x) = x * (4 - x)
t = 2.9 (slider from -5 to 5, step 0.1)
h = 0.39 (slider from 0.01 to 1, step 0.01)
P: (t, f(t))
Q: (t + h, f(t + h))
O = (0, 0)
u = Vector(O, P)
v = Vector(O, Q)
w = Vector(P, Q)
z = Vector(P, P + w / h)