a)
\(x^2\left(x-7\right)=0\)
\(\Leftrightarrow x^2=0;x-7=0\)
\(\Leftrightarrow x=0;x=7\)
b)
\(\left(x+3\right)^2\left(-6-2x\right)^3=0\)
\(\Leftrightarrow\left(x+3\right)^2=0;\left(-6-2x\right)^3=0\)
\(\Leftrightarrow x+3=0;-6-2x=0\)
\(\Leftrightarrow x=-3\)