How to measure the angle of the tangent to an ellipse? If we consider the ellipse in the picture here

How do we determine the angle $\lambda$ of the vector v (tangent at point x = 2 ,y = 3) with the line joining the center (10,0)and the point (2,3)? 
Edit:
a = 10, b = 5
it seems impossible it is 80°, it looks more like 56-57°

$$ \dfrac{x^2}{a^2}+ \dfrac{y^2}{b^2} =1 $$
Differentiate to find slope
$$ y^{'}= \dfrac{x\,  b^2 }{y\, a^2 }= \dfrac{8\, 5^2 }{3 \,10^2} = \dfrac23 $$
Find arctan of above and add to $ {20.55}^0  $ to get correct angle.
