а)

[tex] {x}^{2} - 4x + 3 = 0 \\ x _{1} + x _{2} = 4 \\ x _{1} \times x _{2} = 3 \\ x _{1} = 3 \\ x _{2} = 1[/tex]

б)

[tex] {x}^{2} + 6x + 9 = 0 \\ (x + 3) {}^{2} = 0 \\ x + 3 = 0 \\ x = - 3[/tex]

в)

[tex]4 {x}^{2} - 9x + 15 = 0 \\ d = ( - 9) {}^{2} - 4 \times 4 \times 15 = \\ 81 - 240 = - 159 \\ d < 0[/tex]

нет корней