$\frac{ 18+3\left( a-6 \right)-2a }{ 6a }$
Distribute $3$ through the parentheses$\frac{ 18+3a-18-2a }{ 6a }$
Since two opposites add up to $0$, remove them from the expression$\frac{ 3a-2a }{ 6a }$
Collect like terms$\frac{ a }{ 6a }$
Cancel out the common factor $a$$\begin{align*}&\frac{ 1 }{ 6 } \\&\begin{array} { l }0.1\overset{ \cdot }{ 6 } ,& {6}^{-1}\end{array}\end{align*}$