[tex]\it \overline{AB}=4\overline{AE}\Rightarrow \overline{EB} = \dfrac{3}{4}\overline{AB} \\ \\ \overline{CE} = \overline{CB} +\overline{BE} =-\overline{BC} -\overline{EB} =-\overline{AD} -\dfrac{3}{4}\overline{AB} =-\dfrac{3}{4}\overline{AB} +(-1)\overline{AD} \\ \\ \\ a=-\dfrac{3}{4},\ \ \ b=-1[/tex]