Step 1 :
Factor out common factor $ \color{blue}{ b } $:
$$ 3a^3b+15a^2b+2ab+10b = b ( 3a^3+15a^2+2a+10 ) $$Step 2 :
To factor $ 3a^3+15a^2+2a+10 $ we can use factoring by grouping.
Group $ \color{blue}{ 3a^3 }$ with $ \color{blue}{ 15a^2 }$ and $ \color{red}{ 2a }$ with $ \color{red}{ 10 }$ then factor each group.
$$ \begin{aligned} 3a^3+15a^2+2a+10 &= ( \color{blue}{ 3a^3+15a^2 } ) + ( \color{red}{ 2a+10 }) = \\ &= \color{blue}{ 3a^2( a+5 )} + \color{red}{ 2( a+5 ) } = \\ &= (3a^2+2)(a+5) \end{aligned} $$