a) Ta có: \(\left(2x+7\right)^2=9\left(x+2\right)^2\)
\(\Leftrightarrow\left(2x+7\right)^2-9\left(x+2\right)^2=0\)
\(\Leftrightarrow4x^2+28x+49-9\left(x^2+4x+4\right)=0\)
\(\Leftrightarrow4x^2+28x+49-9x^2-36x-36=0\)
\(\Leftrightarrow-5x^2-8x-13=0\)
\(\Delta=\left(-8\right)^2-4\cdot\left(-5\right)\cdot\left(-13\right)=-196\)
Vì \(\Delta< 0\) nên phương trình vô nghiệm
Vậy: \(x\in\varnothing\)