a) $7x\left(x+1\right)-3\left(x+1\right)=0\Rightarrow \left(x+1\right)\left(7x-3\right)=0$

$\Rightarrow [\begin{array}{c}x+1=0\\ 7x+3=0\end{array}\Rightarrow [\begin{array}{c}x=-1\\ x=-\frac{3}{7}\end{array}$

b) 3(x + 8) - x² - 8x = 0

=> 3(x + 8) - (x² + 8x) = 0

=> 3(x + 8) - x(x + 8) = 0

=> (x + 8)(3 - x) = 0 => $[\begin{array}{c}x+8=0\\ 3-x=0\end{array}\Rightarrow [\begin{array}{c}x=-8\\ x=3\end{array}$