a^2 / (a + b) = d^2 / (c + d), where a & d are unknown, and b & c are constants.

I would like to solve for a in terms of b, c, and d (isolate a in this equality).

This is part of a larger problem I'm working on, but I'm completely stuck right here and drawing a blank on how to handle this.

Any help would be appreciated. Thanks!