You have arctangents. These are equal to angles. For instance, arctan(x) =
for some angle
. Then
. Note that you have a numerical value for x.
Pick an angle variable for arctan(y), and plug the sum of these two angles, being the sum of the arctangents, into the angle-sum formula for tangent. Simplify the result, and plug in values where you can. I haven't completed the computations, but I believe that this process should lead you to the answer.