How to factor the expression $1+s-t-st$ How should this expression be factored $1+s-t-st$ ? I don't know how to begin & I'm clueless. 
Your help would be much appreciated. Thanks

When $s=-1$, $1+s-t-st=0$ and similarly for $t=1$. So we have either $(1+s)(1-t)$ or $(1+s)(t-1)$. By inspection, $1+s-t-st=(1+s)(1-t)$
