In this example we are multiplying two binomials so FOIL method can be used.
$$ \begin{aligned} \left( \color{blue}{ x+3}\right) \cdot \left( \color{orangered}{ x+6}\right) &= \underbrace{ \color{blue}{x} \cdot \color{orangered}{x} }_{\text{FIRST}} + \underbrace{ \color{blue}{x} \cdot \color{orangered}{6} }_{\text{OUTER}} + \underbrace{ \color{blue}{3} \cdot \color{orangered}{x} }_{\text{INNER}} + \underbrace{ \color{blue}{3} \cdot \color{orangered}{6} }_{\text{LAST}} = \\ &= x^2 + 6x + 3x + 18 = \\ &= x^2 + 6x + 3x + 18 = \\ &= x^2+9x+18; \end{aligned} $$