Here is the problem:
step 1: k(k-1)^2 - k(k-1)
step 2: (k-1) [ k(k-1) - k]
now, the answer says the next line is this,
step 3: (k-1) [(k-1)^2 -1]
I'm confused how it goes from step 2 to step 3. There is something I've forgotten and cannot figure out how to go from step 2 to step 3. thanks.