If $x+y=12$ and $x-y=8$, what is the value of $2x-xy$?
I notice that if I add the two equations, I get $2x=20$, which makes it easy to find $x$. Dividing both sides by $2$, I get $x=10$. Now that I have $x$, I can plug it into either equation to find $y$. For example, using $x+y=12$, I get $10+y=12$, so $y=2$. Now I can plug these values into $2x-xy$ and simplify. I get $2(10)-(10)(2)=20-20=0$.

