511. Explain why it is necessary to reverse the inequality when solving \( \frac{n}{-3}<12 \).

\( \left\{\begin{array}{l}\text { Реши систему неравенств: } \\ (5 x+10)-5 \cdot(x+5)>5 x \\ (x-3)(x+8)<0\end{array}\right. \)

(3) If the point \( (x-5,7-x) \) lies in the second quadrant, then \( x=- \) \( \begin{array}{lll}\text { (a) } 9 & \text { (b) } 3 & \text { (c) } 7\end{array} \)

VII. Find the solution(s) of the following system. \[ \left\{\begin{array}{r}x_{1}+x_{2}+x_{3}+x_{4}=0 \\ 2 x_{1}+2 x_{2}+3 x_{3}+4 x_{4}=0 \\ 2 x_{1}+2 x_{2}+2 x_{3}+2 x_{4}=0 \\ 3 x_{1}+3 x_{2}+4 x_{3}+5 x_{4}=0\end{array}\right. \]

What is the greatest natural number \( x \), so that it leaves remainders 8 and 20 when 308 and 200 are divided by \( \times \) respectively.

Solve the exponential equation: \( 3^{2x} = 81 \)