$\displaystyle a=- \frac{2}{3}$\displaystyle a=- \frac{2}{3}

$\displaystyle a=- \frac{3}{2}$\displaystyle a=- \frac{3}{2}

$\displaystyle a= \frac{3}{2}$\displaystyle a= \frac{3}{2}

$\displaystyle a= \frac{2}{3}$\displaystyle a= \frac{2}{3}