Solve each system of equations using elimination method. \begin{array}{l} 2 x+5 y=20 \\ 6 x-5 y=12 \end{array} \begin{array}{l} 3 p+4 q=-3 \\ -p+4 q=-15 \end{array} \begin{array}{l} -7 a+3 b=15 \\

7 a-6 b=-3 \end{array} \begin{array}{l} u+v=11 \\ -u+v=9 \end{array} \begin{array}{l} 5 r+8 s=3 \\ 4 r+8 s=-4 \end{array} \begin{array}{c} -9 c-2 d=8 \\ -9 c-d=6 \end{array} \begin{array}{l} m-4 n=13 \\ m-6 n=12 \end{array} \begin{array}{l} 3 s+7 t=18 \\ 3 s-4 t=-48 \end{array}