Respuesta :
Explicación paso a paso : Apliquemos la formula general a cada ecuación
<img src="https://tex.z-dn.net/?f=1.-%5Cmathbf%7B3x%5E%7B2%7D%2B8x%2B12%3D0%7D%5C%5C%5C%5C%5Cmathbf%7Ba%3D3%2C%20b%3D8%2C%20c%3D12%7D%5C%5C%5C%5C%5Cmathsf%7Bx%3D%5Cfrac%7B-8%5Cpm%5Csqrt%7B8%5E%7B2%7D-4%283%29%2812%29%7D%7D%7B2%283%29%7D%7D%5C%5C%5C%5C%5Cmathsf%7Bx%3D%5Cfrac%7B-8%5Cpm%5Csqrt%7B64-144%7D%7D%7B6%7D%7D%5C%5C%5C%5C%5Cmathsf%7Bx%3D%5Cfrac%7B-8%5Cpm%5Csqrt%7B-80%7D%7D%7B6%7D%7D%5C%5C%5C%5C%5Cmathsf%7Bx%3D%5Cfrac%7B-8%5Cpm2%5Csqrt%7B20%7Di%7D%7B6%7D%7D%5C%5C%5C%5C%5Cmathbf%7Bx_%7B1%7D%3D-%5Cfrac%7B4%7D%7B3%7D%2B%5Cfrac%7B%5Csqrt%7B20%7Di%7D%7B3%7D%7D%5C%5C%5C%5C%5Cmathbf%7Bx_%7B2%7D%3D-%5Cfrac%7B4%7D%7B3%7D-%5Cfrac%7B%5Csqrt%7B20%7Di%7D%7B3%7D%7D%5C%5C%5C%5C2.-%5Cmathbf%7B4x%5E%7B2%7D%2B20x%2B31%3D0%7D%5C%5C%5C%5C%5Cmathbf%7Ba%3D4%2C%20b%3D20%2C%20c%2C%3D31%7D%5C%5C%5C%5C%5Cmathsf%7Bx%3D%5Cfrac%7B-20%5Cpm%5Csqrt%7B%2820%29%5E%7B2%7D-4%284%29%2831%29%7D%7D%7B2%284%29%7D%7D%5C%5C%5C%5C%5Cmathsf%7B%5Cfrac%7B-20%5Cpm%5Csqrt%7B400-496%7D%7D%7B8%7D%7D%5C%5C%5C%5C%5Cmathsf%7B%5Cfrac%7B-20%5Cpm%5Csqrt%7B-96%7D%7D%7B8%7D%7D%5C%5C%5C%5C%5Cmathsf%7B%5Cfrac%7B-20%5Cpm4%5Csqrt%7B6%7Di%7D%7B8%7D%7D%3D-%5Cfrac%7B5%5Cpm%5Csqrt%7B6%7Di%7D%7B2%7D%7D%5C%5C%5C%5C%5Cmathbf%7Bx_%7B1%7D%3D-%5Cfrac%7B5%7D%7B2%7D%2B%5Cfrac%7B%5Csqrt%7B6%7Di%7D%7B2%7D%7D%5C%5C%5C%5C%5Cmathbf%7Bx_%7B2%7D%3D-%5Cfrac%7B5%7D%7B2%7D-%5Cfrac%7B%5Csqrt%7B6%7Di%7D%7B2%7D%7D" />3.
- <img src="https://tex.z-dn.net/?f=%5Cmathbf%7B2x%5E%7B2%7D-12x%2B21%3D0%7D%5C%5C%5C%5C%5Cmathbf%7Ba%3D2%2C%20b%3D-12%2C%20c%3D21%7D%5C%5C%5C%5C%5Cmathsf%7Bx%3D%5Cfrac%7B12%5Cpm%5Csqrt%7B%28-12%29%5E%7B2%7D-4%282%29%2821%29%7D%7D%7B2%282%29%7D%7D%5C%5C%5C%5C%5Cmathsf%7Bx%3D%5Cfrac%7B12%5Cpm%5Csqrt%7B144-168%7D%7D%7B4%7D%7D%5C%5C%5C%5C%5Cmathsf%7Bx%3D%5Cfrac%7B12%5Cpm%5Csqrt%7B-24%7D%7D%7B4%7D%7D%5C%5C%5C%5C%5Cmathsf%7Bx%3D%5Cfrac%7B12%5Cpm2%5Csqrt%7B6%7Di%7D%7B4%7D%7D%5C%5C%5C%5C%5Cmathbf%7Bx_%7B1%7D%3D3%2B%5Cfrac%7B%5Csqrt%7B6%7Di%7D%7B2%7D%7D%5C%5C%5C%5C%5Cmathbf%7Bx_%7B2%7D%3D3-%5Cfrac%7B%5Csqrt%7B6%7Di%7D%7B2%7D%7D" />
4.
- <img src="https://tex.z-dn.net/?f=%5Cmathbf%7Bx_%7B1%7D%3D-%5Cfrac%7B6%7D%7B7%7D%2B%5Cfrac%7B2%5Csqrt%7B5%7Di%7D%7B7%7D%7D%5C%5C%5C%5C%5Cmathbf%7Bx_%7B2%7D%3D-%5Cfrac%7B6%7D%7B7%7D-%5Cfrac%7B2%5Csqrt%7B5%7Di%7D%7B7%7D%7D" />
6.
- <img src="https://tex.z-dn.net/?f=x_%7B1%7D%3D-7%2B2%5Csqrt%7B6%7Di%5C%5C%5C%5Cx_%7B2%7D%3D-7-2%5Csqrt%7B6%7Di" />
7.
- <img src="https://tex.z-dn.net/?f=x%5E%7B2%7D%2B2a%2Ba%5E%7B2%7D%2Bb%5E%7B2%7D%3D0%5C%5C%5C%5Ca%3D1%2C%20b%3D2a%2C%20c%3Da%5E%7B2%7D%2Bb%5E%7B2%7D%5C%5C%5C%5Cx%3D%5Cfrac%7B-2a%5Cpm%5Csqrt%7B%282a%29%5E%7B2%7D-4%281%29%28a%5E%7B2%7D%2Bb%5E%7B2%7D%29%7D%7D%7B2%281%29%7D%5C%5C%5C%5Cx%3D%5Cfrac%7B-2a%5Cpm%5Csqrt%7B4a%5E%7B2%7D-4a%5E%7B2%7D-4b%5E%7B2%7D%7D%7D%7B2%7D%5C%5C%5C%5Cx%3D%5Cfrac%7B-2a%5Cpm%5Csqrt%7B-4b%5E%7B2%7D%7D%7D%7B2%7D%5C%5C%5C%5Cx%3D%5Cfrac%7B-2a%5Cpm%202bi%7D%7B2%7D%5C%5C%5C%5Cx%3D-a%5Cpm%20bi%5C%5C%5C%5Cx_%7B1%7D%3D-a%2Bbi%5C%5C%5C%5Cx_%7B2%7D%3D-a-bi" />
Saludos.