[tex]\textit{area of a rectangle}\\\\ A=Lw ~\begin{cases} L=length\\ w=width\\[-0.5em] \hrulefill\\ L=\stackrel{cm}{6}\\ A=\stackrel{cm^2}{6x+18} \end{cases}\implies 6x+18=6w \\\\\\ \cfrac{6x+18}{6}=w\implies \cfrac{6x}{6}+\cfrac{18}{6}=w\implies x+3=w[/tex]