X^2+5x+6=0 X^2-6x+5=0 X^2-8x+15=0

1)
${x}^{2} + 5x + 6 = 0 \\ d = 25 - 4 \times 6 = 1 \\ x1 = \frac{ - 5 + 1}{2} = - 2 \\ x2 = \frac{ - 5 - 1}{ 2} = - 3$
2)
$x {}^{2} - 6x + 5 = 0 \\ d = 36 - 4 \times 5 = 36 - 20 = 16 = {4}^{2} \\ x1 = \frac{6 + 4}{2} = 5 \\ x2 = \frac{6 - 4}{2} = 1$
3)
${x}^{2} - 8x + 15 = 0 \\ d = 64 - 4 \times 15 = 4 = {2}^{2} \\ x1 = \frac{8 - 2}{2} = 3 \\ x2 = \frac{8 + 2}{2} = 10$