a/
\(\Leftrightarrow\left(x-1\right)\left(x-2\right)-3\left(x-2\right)=0\)
\(\Leftrightarrow\left(x-2\right)\left(x-4\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=2\\x=4\end{matrix}\right.\)
b/
\(\Leftrightarrow x^2-4+4x^2+4x+1-5x^2+15x=5\)
\(\Leftrightarrow19x=8\)
\(\Rightarrow x=\frac{8}{19}\)