I am struggling to understand how the following is true for the fermionic creation/annihilation operators $a^\dagger, a$: $$[a^\dagger a, a]=-a$$
If someone could walk me through the math derivation of this, I would greatly appreciate it!
I see that $[a^\dagger a, a]= a^\dagger aa - a a^\dagger a = (1-a a^\dagger)a - a a ^\dagger a = a - 2 a a^\dagger a$ but I'm not sure how to simplify this further to get $-a$ at the end.