k = 0.2 (slider from -2 to 4, step 0.1)
inviluppo(x) = (k * x^(2))
prm = -4 (slider from -4 to 4, step 0.5)
PTG: (prm, inviluppo(prm))
ca: ((k * 2) * prm)
rettaPRM: y - y(PTG) = (ca * (x - prm))