2005 AMC 12A Problem 9

There are two values of a for which the equation 4x^2 + ax + 8x + 9 = 0 has only one solution for x. What is the sum of those values of a?

\textbf{(A) } -16 \qquad \textbf{(B) } -8 \qquad \textbf{(C) } 0 \qquad \textbf{(D) } 8 \qquad \textbf{(E) } 20

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.