More stuff from my textbook that I'm not quite understanding, help is appreciated. I'll be trying to figure it out and updating my question as I wait for answers.
I understand how $3k^2+3k$ factors to become $3(k^2+k)$, but what's happening to the $+1$ tacked on at the end?
EDIT: I see about the $+1$, since we're subtracting $k+1$, that means that the $+1$ is being removed from our left side equation.