In this example we are multiplying two binomials so FOIL method can be used.
$$ \begin{aligned} \left( \color{blue}{ 3x+3}\right) \cdot \left( \color{orangered}{ 3x+3}\right) &= \underbrace{ \color{blue}{3x} \cdot \color{orangered}{3x} }_{\text{FIRST}} + \underbrace{ \color{blue}{3x} \cdot \color{orangered}{3} }_{\text{OUTER}} + \underbrace{ \color{blue}{3} \cdot \color{orangered}{3x} }_{\text{INNER}} + \underbrace{ \color{blue}{3} \cdot \color{orangered}{3} }_{\text{LAST}} = \\ &= 9x^2 + 9x + 9x + 9 = \\ &= 9x^2 + 9x + 9x + 9 = \\ &= 9x^2+18x+9; \end{aligned} $$