Cho tứ giác ABCD. Tìm M:
a) \(\overrightarrow{MA}+2\overrightarrow{MB}+3\overrightarrow{MC}=\overrightarrow{0}\)
b) \(2\overrightarrow{MA}+\overrightarrow{MB}+\overrightarrow{MC}+3\overrightarrow{MD}=\overrightarrow{0}\)
c) \(\overrightarrow{MA}+3\overrightarrow{MB}-\overrightarrow{MC}+\overrightarrow{MD}=\overrightarrow{0}\)