For this you can use the equation point slope, that is,
[tex]\begin{gathered} y-y_1=m(x-x_1) \\ \end{gathered}[/tex]Where,
[tex]\begin{gathered} m\colon\text{ slope of the line} \\ (x_{1,}y_1_{})\colon\text{ coordinates of the given point through which the line passes} \end{gathered}[/tex]So, you have
[tex]\begin{gathered} y-(-3)=3(x-3) \\ y+3=3x-9 \\ \text{ Substract 3 from both sides of the equation} \\ y+3-3=3x-9-3 \\ \text{Therefore, the equation of the line is} \\ y=3x-12 \end{gathered}[/tex]