Mn giúp mik bài này với ạ . Mn chân thành cảm ơn mn !

$\begin{array}{l}a)\,\dfrac{x+1}{3} = – \dfrac{15}{7}\\ \Leftrightarrow x + 1 = -\dfrac{15}{7}\cdot 3\\ \Leftrightarrow x + 1 = – \dfrac{45}{7}\\ \Leftrightarrow x = – \dfrac{45}{7} – 1\\ \Leftrightarrow x = – \dfrac{52}{7}\\ b)\,\dfrac{x +2}{25} = \dfrac{-100}{2+ x}\qquad (x \ne – 2)\\ \Leftrightarrow (x+2)^2 = -100.25\\ \Leftrightarrow (x+2)^2 = -2500 \quad (vô\,\,lí)\\ \Rightarrow \text{Phương trình vô nghiệm}\\ c)\,\dfrac{x+5}{4} = \dfrac{x}{-13}\\ \Leftrightarrow -13(x+5) = 4x\\ \Leftrightarrow -13x – 75 = 4x\\ \Leftrightarrow 17x = -75\\ \Leftrightarrow x = – \dfrac{75}{17}\\ d)\,\dfrac{3x – 21}{9} = – \dfrac{9}{7-x}\qquad (x \ne 7)\\ \Leftrightarrow \dfrac{x – 7}{3} = \dfrac{-9}{7-x}\\ \Leftrightarrow \dfrac{x-7}{3} = \dfrac{9}{x – 7}\\ \Leftrightarrow (x-7)^2 = 27\\ \Leftrightarrow \left[\begin{array}{l}x – 7 = 3\sqrt3\\x – 7 = – 3\sqrt3\end{array}\right.\\ \Leftrightarrow \left[\begin{array}{l}x =7+ 3\sqrt3\\x =7 – 3\sqrt3\end{array}\right.\\ \end{array}$