a: \(\dfrac{-2}{x^2+6x}=\dfrac{-2}{x\left(x+6\right)}\)
\(\dfrac{x}{x+6}=\dfrac{x\cdot x}{x\left(x+6\right)}=\dfrac{x^2}{x\left(x+6\right)}\)
b: \(\dfrac{11}{25-x^2}=\dfrac{-11}{x^2-25}=\dfrac{-11}{\left(x-5\right)\left(x+5\right)}=\dfrac{-11\left(x-5\right)}{\left(x-5\right)^2\cdot\left(x+5\right)}\)
\(\dfrac{2}{x^2-10x+25}=\dfrac{2}{\left(x-5\right)^2}=\dfrac{2\left(x+5\right)}{\left(x+5\right)\cdot\left(x-5\right)^2}\)