Answer:
d = 7
For step by step explaintion I am so sorry but I don't rember enough of my geometry class to properly explain this, but triangles BDC and ABD are congruent (I swear I can prove this if I dig up my old notes but I really do not feel like doing that) so these 2 equations (sides) are equal. So we just solve for d!
6d + 3 = 10d - 25
(we all know how to solve something this simple so I won't bore you with the details of inverse operations and such)
d = 7