a, \(x\ne-1;3\)
b, Ta có : \(P=\frac{3x^2+3x}{\left(x+1\right)\left(2x-6\right)}=1\)
\(\Leftrightarrow\frac{3x\left(x+1\right)}{\left(x+1\right)\left(2x-6\right)}=1\Leftrightarrow\frac{3x}{2\left(x-3\right)}=1\)
\(\Leftrightarrow3x=2x-6\Leftrightarrow x=-6\)