Lời giải:
a)
\((x+3)^2=x+3\)
\(\Leftrightarrow (x+3)^2-(x+3)=0\)
\(\Leftrightarrow (x+3)(x+3-1)=0\)
\(\Leftrightarrow (x+3)(x+2)=0\Rightarrow \left[\begin{matrix} x=-3\\ x=-2\end{matrix}\right.\)
b) \((x-1)(x+2)=x^2-8\)
\(\Leftrightarrow x^2+2x-x-2=x^2-8\)
\(\Leftrightarrow x=-6\)