[tex] {x}^{2} - mx + 4 = 0 \\ x_{1} = - 0.8 \\ ( - 0.8) {}^{2} - ( - 0.8)m + 4 = 0 \\ 0.64 + 0.8m + 4 = 0 \\ 0.8m + 4.64 = 0 \\ 0.8m = - 4.64 \\ m = - 4.64 \div 0.8 \\ m = - 5.8[/tex]

Получается уравнение:

[tex] {x}^{2} - ( - 5.8)x + 4 = 0 \\ {x}^{2} + 5.8x + 4 = 0[/tex]

Ответ: m = - 5,8