\[x - 8 = 0\]
\[x = 8.\]
\[D = ( - 3)^{2} - 4 \cdot 1 \cdot ( - 40) =\]
\[= 9 + 160 = 169\]
\[x_{1} = \frac{3 + \sqrt{169}}{2 \cdot 1} = \frac{3 + 13}{2} =\]
\[= \frac{16}{2} = 8\]
\[x_{2} = \frac{3 - \sqrt{169}}{2 \cdot 1} = \frac{3 - 13}{2} =\]
\[= - \frac{10}{2} = - 5\]
\[Ответ:\ x = 8.\]