[tex]\bf \begin{cases}
f(x)=4x+3\\
g(x)=3x\\
------------\\
f(x)\cdot g(x)=(4x+3)(3x)\\
\qquad \qquad \qquad 12x^2+9x
\end{cases}
\\\\\\
\boxed{x=-\cfrac{1}{2}}\qquad \qquad 12\left( -\frac{1}{2} \right)^2+9\left( -\frac{1}{2} \right)\implies 12\left( -\frac{1}{2} \right)\left( -\frac{1}{2} \right)+9\left( -\frac{1}{2} \right)
\\\\\\
12\left( \cfrac{1}{4} \right)-\cfrac{9}{2}\implies 3-\cfrac{9}{2}\implies \cfrac{6-9}{2}\implies \cfrac{-3}{2}\implies -1\frac{1}{2}[/tex]
