First we need to check where the function gets undefined.
So solve it now
[tex]\\ \tt\hookrightarrow -3x^2+3x+18=0[/tex]
[tex]\\ \tt\hookrightarrow -3(x^2-x-6)=0[/tex]
[tex]\\ \tt\hookrightarrow x^2-x-6=0[/tex]
[tex]\\ \tt\hookrightarrow x^2+2x-3x+6=0[/tex]
[tex]\\ \tt\hookrightarrow x(x+2)-3(x+2)=0[/tex]
[tex]\\ \tt\hookrightarrow (x-3)(x+2)=0[/tex]
[tex]\\ \tt\hookrightarrow x=3\;or\:x=-2[/tex]