I would probably rearrange the equation and continue removing the squares.
and so on. Then you'll be left with a quartic equation which you can work with.
I put the to WolframAlpha so that I can see how this function looks like and.. there are no answers. I mean - few of them are saying that l or k shoud be 0 which doesn't fit the requirements of the problem and the others are so extraordinarily long I seriously doubt they may be correct. What am I doing wrong?