\(\left\{\begin{array}{ccc}x=2y\\2x+2y=24&|divide\ both\ sides\ by\ 2\end{array}\right\\\left\{\begin{array}{ccc}x=2y\\x+y=12\end{array}\right\\\\subtitute\ x=2y\ to\ the\ x+y=12\\\\2y+y=12\\3y=12\ \ \ |divide\ both\ sides\ by\ 3\\\boxed{y=4}\\\\x=2y\to x=2\cdot4\\\boxed{x=8}\\\\Answer:\fbox{a.}\)