Show tangent to f at A

a = 1.3 (slider from -5 to 5, step 0.1)
b = 0.3 (slider from -5 to 5, step 0.1)
c = -3.8 (slider from -5 to 5, step 0.1)
f(x) = a * x^(2.0) + b * x + c
s(x) = 2.0 * a * x + b
xA = 2.2 (slider from -5 to 5, step 0.1)
A: (xA, f(xA))
t(x) = s(x(A)) * (x - x(A)) + y(A)
showt = false