1.

х² + 11х - 12 = 0,

Д = 11² - 4*1*(-12) = 121 + 48 = 169,

х1 = (-11 + 13) / 2*1 = 2/2 = 1,

х2 = (-11 - 13) / 2*2 = -24/2 = -12,

2.

х² - 20х = 0,

х * (х - 20) = 0,

х1 = 0,    х - 20 = 0,

              х2 = 20,

3.

-5х + 4(х - 6) ≥ 0,

-5х + 4х - 24 ≥ 0,

-х - 24 ≥ 0,

-х ≥ 24,

х ≤ -24