f(x) = x^(2)
x_0 = 0.9 (slider from -5 to 5, step 0.1)
P_0: (x_0, f(x_0))
h = 0.443 (slider from -1 to 1, step 0.001)
P: (x_0 + h, f(x_0 + h))
a = Line(P_0, P)
k = Slope(a)