\[\boxed{\text{1116.\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
\[\textbf{а)}\ f(x) = - 0,7x + 350\]
\[- 0,7x + 350 = 0\]
\[- 0,7x = - 350\]
\[x = 500 - нули\ функции.\]
\[f(x) > 0:\]
\[при\ x < 500.\]
\[f(x) < 0:\]
\[при\ x > 500.\]
\[\textbf{б)}\ f(x) = 30x + 10\]
\[30x + 10 = 0\]
\[30x = - 10\]
\[x = - \frac{1}{3} - нули\ функции.\]
\[f(x) > 0:\]
\[при\ x > - \frac{1}{3}.\]
\[f(x) < 0:\]
\[при\ x < - \frac{1}{3}.\]