[tex] {x}^{2} + 15 + a = 0 \\ x _{1} = - 12 \\ ( - 12) {}^{2} + 15 + a = 0 \\ 144 + 15 + a \\ 159 + a = 0 \\ a = - 159 \\ \\ {x}^{2} + 15 - 159 = 0 \\ {x}^{2} - 144 = 0 \\ (x - 12)(x + 12) = 0 \\ x _{1} = - 12 \\ x _{2} = 12[/tex]
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[tex] {x}^{2} + 15 + a = 0 \\ x _{1} = - 12 \\ ( - 12) {}^{2} + 15 + a = 0 \\ 144 + 15 + a \\ 159 + a = 0 \\ a = - 159 \\ \\ {x}^{2} + 15 - 159 = 0 \\ {x}^{2} - 144 = 0 \\ (x - 12)(x + 12) = 0 \\ x _{1} = - 12 \\ x _{2} = 12[/tex]