d²+12d+36 can written under the form of ax²+bx+c or ad²+bd+c in this case. Also, to factor this out, we can change ad²+bd+c to ad²+md+nd+c. Like this:
ad²+bd+c
=ad²+md+nd+c
d²+12d+36
=d²+6d+6d+36
=d(d+6)+6(d+6)
=(d+6)(d+6) or (d+6)² is your factor form of d²+12d+36. Hope it help!