a, \(\left(2x-1\right)\left(x+3\right)-2x^2+5x=7\)
\(\Leftrightarrow2x^2+6x-x-3-2x^2+5x=7\)
\(\Leftrightarrow2x^2+5x-3-2x^2+5x=7\)
\(\Leftrightarrow10x-10=0\Leftrightarrow x=1\)
b, \(\left(x+3\right)\left(x^2-3x+9\right)-x\left(x-4\right)\left(x+4\right)=54\)
\(\Leftrightarrow\left(x^3+27\right)-x\left(x^2-16\right)=54\)
\(\Leftrightarrow x^3+27-x^3+16x=54\)
\(\Leftrightarrow-27+16x=0\Leftrightarrow x=\frac{27}{16}\)