\(a,9x^2-1=0\)
\(\left(3x\right)^1-1=0\)
\(\left(3x-1\right)\cdot\left(3x+1\right)=0\)
\(\hept{\begin{cases}3x-1=\\3x+1=0\end{cases}\Rightarrow\hept{\begin{cases}x=\frac{1}{3}\\x=-\frac{1}{3}\end{cases}}}\)
\(b,x\cdot\left(x+5\right)-x-5=0\)
\(x\cdot\left(x+5\right)-\left(x+5\right)=0\)
\(\left(x+5\right)\cdot\left(x-1\right)=0\)
\(\hept{\begin{cases}x+5=0\\x-1=0\end{cases}\Rightarrow\hept{\begin{cases}x=-5\\x=1\end{cases}}}\)