If you are given a vector as the sum of two angles equaling another angle, say <a+<b = <c. You are given <a, <c and x-y tilt angle <d, where <a is an angle on the x-y plane and the <b plane has a given tilt from the x-y plane of <d. How do you find the angle <b?

(This is an example of a start of elliptical orbit around the sun. <a would be the Longitude of the Ascending Node. <b would be the Argument of the Perihelion. <c would be the Longitude of the Perihelion. And <d would be the Incline.)