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[tex]\large\boxed{\begin{array}{l}\sf f(x)=x^2+4x-21\\\sf f(x)=0\longrightarrow x^2+4x-21=0\\\sf\Delta=b^2-4ac\\\sf\Delta=4^2-4\cdot1\cdot(-21)\\\sf\Delta=16+84\\\sf\Delta=100\\\sf x=\dfrac{-b\pm\sqrt{\Delta}}{2a}\\\sf x=\dfrac{-4\pm\sqrt{100}}{2\cdot1}\\\sf x=\dfrac{-4\pm10}{2}\begin{cases}\sf x_1=\dfrac{-4+10}{2}=\dfrac{6}{2}=3\\\sf x_2=\dfrac{-4-10}{2}=-\dfrac{14}{2}=-7\end{cases}\end{array}}[/tex]
