Step 1: First we have to write polynomials in descending order.

$$ \begin{aligned} P(x) &= x^2+3x \\ Q(x) &= 6x+1 \\ \end{aligned} $$

In this example we are multiplying two binomials so FOIL method can be used.

$$ \begin{aligned} \left( \color{blue}{ x^2+3x}\right) \cdot \left( \color{orangered}{ 6x+1}\right) &= \underbrace{ \color{blue}{x^2} \cdot \color{orangered}{6x} }_{\text{FIRST}} + \underbrace{ \color{blue}{x^2} \cdot \color{orangered}{1} }_{\text{OUTER}} + \underbrace{ \color{blue}{3x} \cdot \color{orangered}{6x} }_{\text{INNER}} + \underbrace{ \color{blue}{3x} \cdot \color{orangered}{1} }_{\text{LAST}} = \\ &= 6x^3 + x^2 + 18x^2 + 3x = \\ &= 6x^3 + x^2 + 18x^2 + 3x = \\ &= 6x^3+19x^2+3x; \end{aligned} $$

Polynomial Operations Calculator