First part of the question was easy,

The second part is where i'm stuck.

What I have tried :

-Make d the subject in both cases and have one equal to each other (All that happens is that everything cancels each other out and I get 0=0)

-Make d^2 the subject (I don't know why I tried this, same result as before >.>)

Completely lost as to how to solve this problem.

Any help would be appreciated!

(Sorry for poor diagram etc. in advance)

Click the image to make it larger.