$2n-6+\left( n-6 \right)+n=45$
When there is a $+$ in front of an expression in parentheses, the expression remains the same$2n-6+n-6+n=45$
Collect like terms$4n-6-6=45$
Calculate the difference$4n-12=45$
Move the constant to the right-hand side and change its sign$4n=45+12$
Add the numbers$4n=57$
Divide both sides of the equation by $4$$\begin{align*}&n=\frac{ 57 }{ 4 } \\&\begin{array} { l }n=14 \frac{ 1 }{ 4 },& n=14.25\end{array}\end{align*}$