Given the geometry shown below I need to find angle r. I now all the sides (a,b,c,d). I do NOT know what angle K is. Here is the geometry setup:

I approached this problem the following way:

EC = AB + 2*BC*sin(k)

DE^2 = DC^2 + EC^2 - 2*DC*EC*cos(r)

I have two equations and three unknowns. Which means I am missing one more equation. Any suggestions? Or maybe a better approach?

Thanks.