Could do with some maths help for calculating an angle.

So, I've got a plane, which is dipping at a known angle. I basically want to calculate what the angle would be if I was looking down the angle, obliquely. As in, if i was looking directly down the slope it would be angle x, if i was looking along the slope it would be 0, and i need to know what it would be at a different, given angle (other arrow on the diagram).

Thanks!