Help to solve a system equation: $x-y+xy=-4$; $xy(x-y)=-21$. I need to solve a system equation. Here's how it looks:
$x-y+xy=-4$
$xy(x-y)=-21$
I tried to substitute $x-y$ with $w$ and $xy$ with $t$ to simplify everything. After that I got this system equation:
$w+t=-4$
$tw=-21$
I solved this new system equation and got these results: 
$t_1=-7, w_1=3$
$t_2=3, w_2=-7$
After all these steps I ended up with 2 new system equations:
1.
$xy=-7 (\leftarrow t)$
$x-y=3 (\leftarrow w)$
2.
$xy=3 (\leftarrow t)$
$x-y=-7 (\leftarrow w)$
Looks like the first one doesn't have any solutions. And I can't solve the second one. Am I doing wrong steps? Please help me to solve this system equation. Thanks.

HINTS: 
We have the equations $$x-y+xy=-4\implies x-y=-4-xy\tag{1}$$ and $$xy(x-y)=-21\tag{2}$$ so substituting $(1)$ into $(2)$, we get $$xy(-4-xy)=-21\implies(xy)^2+4xy-21=0$$ Let $u=xy$. Then $$u^2+4u-21=0$$ and so...
