$a)$

$b)$

$c)$ $\left(k-1\right)\left(m-1+m+1\right)=\left(k-1\right)2m$

$\left(a-1\right)(b+c-\left(b-c\right)=\left(a-1\right)\left(b+c-b+c\right)=2c\left(a-1\right)$