Is the Matrix Nilpotent? 
Let $A, B$ be nilpotent matrices over $\Bbb C$ where $AB=BA$. Are the following matrices nilpotent $$A-2B $$
$$A^3B$$

I don't really  know where to start here. I saw a solution with the binomial expansion, however it seemed hard to understand.
Is there a different way?
Thanks,
Alan

Suppose that $A^a = 0$ and that $B^b=0$ for some integers $a,b\geq 1$ (i.e. that $A$ and $B$ are nilpotent), and furthermore that $AB=BA$ (i.e. that they commute).
You have then that $(A^3B)^{ab} = A^{3ab}B^{ab} = (A^a)^{3b}(B^b)^a = 0^{3b}0^a=0$
(Note: we could have used a smaller exponent than $ab$ and gotten a similar result, but it is unimportant.  All we needed was to show that $A^3B$ is nilpotent, but we didn't care what the smallest such power is that works.  An exponent as small as $\min\{a,b\}$ would have sufficed.)
(Also note: the step $(A^3B)^k = A^{3k}B^k$ is only valid if $A$ and $B$ commute.  This is not true in general.)
As for $(A-2B)$, we approach similarly and use a weaker form of the binomial expansion argument that hopefully you will understand better than the solution you saw earlier.
$(A-2B)^{a+b} = \underbrace{(A-2B)(A-2B)(A-2B)\cdots (A-2B)}_{a+b~\text{number of times}}$
If you expand the right-hand side by "foiling", you will see that every term will be of the form $A^i(-2B)^j$ where $i+j=a+b$.  You will have $i\geq a$ or $j\geq b$.
In the first case, $A^i$ will be zero then, and any matrix times a zero matrix will be zero.  In the second case, $(-2B)^j$ will be zero, and the product will again be zero.
As a result, all terms in the expansion of $(A-2B)^{a+b}$ will be zero, and so $(A-2B)^{a+b}=0$, showing its nilpotence.
