$ y=x^2 $

y=0

y=ax^{2}

a = -2.25 (slider from -4 to 4, step 0.25)
f(x) = (a * x^(2))
O = (0, 0)
a' = FractionText(a)
testo1: "$ y=" + (LaTeX[a']) + "x^2 $"