[tex]15. \: log(4) + log(- x) = log(32) \\\Leftrightarrow log( - x) = log(32) - log(4) \\ \Leftrightarrow log( - x) = log(8) \\ \Leftrightarrow - x = 8 \\ \Leftrightarrow x = - 8 \\ 16. \: log_{4}(5x) - log_{4}(8) = log_{4}(5) \\ \Leftrightarrow log_{4}( \frac{5x}{8} ) = log_{4}(5) \\ \Leftrightarrow \frac{5x}{8} = 5\Leftrightarrow 5x = 40\Leftrightarrow x = 8[/tex]
