It's late and I really am at an end on this, I just can't seem to get it.

Note: A+B = C+D

So, I need to find B in terms of A.

A+B / A-B = k1/k2 * C+D / C-D

cross multiply

(A+B)k2 / (A-B)k1 = C+D / C-D

but A+B = C+D

so

(A+B)k2 / (A-B)k1 = A+B / C-D

=> (A+B)k2 / (A-B)k1) / (A+B) = C-D

(A+B)k2 / (A-B)k1 * (A+B) = C-D

Aagh!

Sorry, I can't do more, please help