Question

X ao quadrado menos y ao quadrado = -120.
X + y = 20

Answer 1

$\large\boxed{\begin{array}{l}\rm x^2-y^2=-120\\\rm (x-y)\cdot(x+y)=-120\\\rm (x-y)\cdot20=-120\\\\\rm x-y=-\dfrac{120}{20}\\\\\rm x-y=-6\end{array}}$

$\large\boxed{\begin{array}{l}+\underline{\begin{cases}\rm x+\diagdown\!\!\!\!y=20\\\rm x-\diagdown\!\!\!\!y=-6\end{cases}}\\\rm 2x=14\\\rm x=\dfrac{14}{2}\\\\\rm x=7\\\rm x+y=20\\\rm 7+y=20\\\rm y=20-7\\\rm y=13\end{array}}$