$\displaystyle a= \frac{L-2b-c}{2}$\displaystyle a= \frac{L-2b-c}{2}

$\displaystyle a= \frac{L-2b+c}{2}$\displaystyle a= \frac{L-2b+c}{2}

$a=L+2b-c$a=L+2b-c

$a=L-2b-c$a=L-2b-c