\[y = x^{2} - 6x + 5;\ \ y = 21\]
\[x^{2} - 6x + 5 = 21\]
\[x^{2} - 6x - 16 = 0\]
\[D_{1} = 9 + 16 = 25 > 0 - два\ решения.\]
\[Ответ:В.\]