FERMAT 2006 Problem 14

If the system of equations

\begin{aligned} & p x+q y=8 \\ & 3 x-q y=38 \end{aligned}

has the solution (x,y)=(2,-4), then p is equal to

Answer Choices
A. -12
B. 20
C. 8
D. 40
E. 21.5