[tex]\bf \begin{cases}
x^2-y^2=120\\
x-y=10\implies \boxed{x}=10+y\\
----------\\
\left( \boxed{10+y} \right)^2-y^2=120
\end{cases}
\\\\\\
(10^2+20y+y^2)~~-~~y^2=120\implies 100+20y=120
\\\\\\
20y=20\implies y=\cfrac{20}{20}\implies y=1\\\\
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x-y=10\implies x-(1)=10\implies x=11[/tex]
