Нужно решить 7,8,9,10

Решите задачу:

$7)\quad 5\cdot 10^{lgx}=7x+15\; ,\quad ODZ:\; x\ \textgreater \ 0\\\\5x=7x+15\; ;\; \; 2x=-15\; ;\; \; x=-7,5\ \textless \ 0\\\\Otvet:\; net\; \; reshenij\\\\8)\quad cosa=-\sqrt{\frac{5}{21}}\; ,\; \; \pi \leq a \leq \frac{3\pi}{2}\; \to \; sina\ \textless \ 0\\\\\sqrt{21}sina=\sqrt{21}\cdot (-\sqrt{1-cos^2a})=-\sqrt{21}\cdot \sqrt{1-\frac{5}{21}}=\\\\=-\sqrt{21}\cdot \frac{4}{\sqrt{21}}=-4$

$9)\quad \frac{(x-2)(x+4)}{x-1} \leq 0\\\\---[-4]+++(1)---[2]+++\\\\x\in (-\infty ,-4\, ]\cup (1,2\, ]\\\\10)\quad log_7(8x-20)-log_72=log_73\; ,\quad ODZ:\; x\ \textgreater \ \frac{5}{2}\\\\ \frac{8x-20}{2} =3\\\\8x-20=6\\\\8x=26\\\\x=3,25$