Tìm x biết rằng

$$\begin{gathered} Tìm x biết \\ a. {\left(\begin{array}{cc}3x+& 4\end{array}\right)}^2-\left(\begin{array}{cc}3x-& 1\end{array}\right)\left(\begin{array}{cc}3x+& 1\end{array}\right)=49 \\ b. x^2-4x+4=9\left(\begin{array}{cc}x-& 2\end{array}\right) \end{gathered}$$