Given that overrightarrow{O A}=11 x+6 y, \overrightarrow{O B}=4 x+10 y and \overrightarrow{C O}=-13 x+11 y, write down each of the following vectors in its simplest form.
a) $\overrightarrow{B A}=
b) $\overrightarrow{A C}=