a>
5x - 25x - $x^{2}$ -5x= 0
- 25x - $x^{2}$= 0
x(-25-x)=0
\(\left[ \begin{array}{l}x=0\\-25 - x=0 => -x=25=> x= -25\end{array} \right.\)
b>
2x( x-2 ) = x-2
2x( x-2 ) - (x-2) = 0
(x-2)(2x-1)=0
\(\left[ \begin{array}{l}x-2=0 => x=2\\2x-1=0 => 2x=1 => x=\frac{1}{2} \end{array} \right.\)