In step two below
I'd say the derivative of $-6x$ is $-6$, and so I'd multiple the $-6$ with the numerator to get $-6$.
But I don't understand where $36$ comes from.
(I understand that $-6$ times $-6$ is $36$, of course, but why it's happened, I don't know, because multiplying a fraction by a scalar should only change the numerator, in my mind)
I assume that I'm missing a step, but I can't think what step. Could you enlighten me?