a) 3x(x+2)-(x+2)=0
(x+2)(3x-1)=0
\(\Leftrightarrow\left[{}\begin{matrix}x=-2\\x=\dfrac{1}{3}\end{matrix}\right.\)
b) \(\left(x-5\right)^2-9x^2=\left(x-5-3x\right)\left(x-5+3x\right)=\left(2x+5\right)\left(5-4x\right)\)
c) x-10x+25=0
-9x+25=0
\(x=\dfrac{25}{9}\)