[tex]\sf 7 {x}^{2} + 14x - 21 = 0[/tex]
[tex]\sf \Delta = {b}^{2} - 4ac[/tex]
[tex]\sf \Delta = {14}^{2} - 4\cdot 7\cdot ( - 21)[/tex]
[tex]\sf \Delta = 196 - 4\cdot 7\cdot ( - 21)[/tex]
[tex]\sf \Delta = 196 + 588[/tex]
[tex]\sf \Delta = 784[/tex]
[tex]\sf x_{½} = \dfrac{ - b \pm \sqrt{\Delta} }{2a} [/tex]
[tex]\sf x_{½} = \dfrac{ - 14 \pm \sqrt{784} }{2\cdot 7} [/tex]
[tex]\sf x_{½} = \dfrac{ - 14 \pm 28 }{14} [/tex]
[tex]\sf \red{x_{1} = \dfrac{ - 14 - 28 }{ 14} = \dfrac{ - 42}{ \: \: \: \: 14} = - 3}[/tex]
[tex]\sf\red{ x_{2} = \dfrac{ - 14 + 28 }{ 14} = \dfrac{ 14}{ 14} = 1}[/tex]
