Commutator relation identity for Quantum mechanics The question asked to show that the identity holds 
$\left [ \hat{a}_{-},\left ( \hat{a}_{+} \right ) \right ]=n\left ( \hat{a}_{+} \right )^{n-1}$ between operators.
The solution that came with the tutorial were 'ugly' and a stark deviation in the flavour in my attempt. However, my attempt did not result in showing the identity despite the fact that-to the best of my knowledge-no error has been committed. 
Attempt:
$\left [ \hat{a}_{-},\left ( \hat{a}_{+} \right ) \right ]=\hat{a}_{-}\left ( \hat{a}_{+} \right )^{n}-\left ( \hat{a}_{+} \right )^{n}\hat{a}_{-}
=\hat{a}_{-}\hat{a}_{+}\cdot \cdot \cdot \hat{a}_{+}-\hat{a}_{-}\cdot \cdot \cdot \hat{a}_{-}\hat{a}_{+} $
$\left ( n\text{ factor for } \hat{a}_{+} \right )$
$=\left ( \hat{a}_{-}\hat{a}_{+} \right )\left ( \hat{a}_{+}\cdot \cdot \cdot \hat{a}_{+} \right )-\left ( \hat{a}_{+}\cdot \cdot \cdot \hat{a}_{+} \right )\left ( \hat{a}_{+}\hat{a}_{-} \right )$
$\left ( n-1\text{ factor for } \hat{a}_{+} \right )$
$=\left ( \left ( \hat{a}_{-}\hat{a}_{+} \right )-\left ( \hat{a}_{+}\hat{a}_{-} \right ) \right )\left ( \hat{a}_{+}\cdot \cdot \cdot \hat{a}_{+} \right )$
=$\left ( \hat{a}_{+} \right )^{n-1}$
Where's the $n$?

you mistke is after the line
$$(a_-a_+)(a_+\dots a_+)-(a_+\dots a_+)(a_+a_-)$$
which is not equal to $(a_-a_+ - a_+a_-)(a_+\dots a_+)$, because $a_+a_-$ does not commute with $a_+$.
Hint for the correct solution: You need to use induction over $n$, i.e. reduce $[a_-,a_+^n]$ to some term involving $[a_-,a_+^{n-1}]$
