The variable a is the lower 
limit of the integral in 
$\frac{dF}{dx}=\frac{d}{dx}\int_a^{x}f(t)dt=f(x).$

a = 2.2 (slider from -5 to 5, step 0.1)
f(x) = x^(2.0) + x^(3.0)
g = Integral(f(x))
h(x) = g(x) - g(a)
A = Point(h)
b = Tangent(A, h(x))
B: (x(A), Slope[b])