The graph of 2 x^{2}+x y+3 y^{2}-11 x-20 y+40=0 is an ellipse in the first quadrant of the x y-plane. Let a and b be the maximum and minimum values of \dfrac{y}{x} over all points (x, y) on the ellipse. What is the value of a+b ?
Answer Choices
A. 3
B. \sqrt{10}
C. \dfrac{7}{2}
D. \dfrac{9}{2}
E. 2 \sqrt{14}