[tex]\bf \stackrel{\stackrel{\textit{net income}}{\textit{revenue}}}{f(x)}=9x^2-54x-144\implies \stackrel{\textit{revenue}}{0}=9x^2-54x-144 \\\\\\ 0=9(x^2-6x-16)\implies 0=x^2-6x-16 \\\\\\ 0=(x+2)(x-8)\implies x= \begin{cases} -2\\ 8&\checkmark \end{cases}[/tex]
since the units sold can't be negative, we can't use -2.
