C# - Find Y coordinate given X value and slope (in degrees)

Hello,

First time poster, pretty stumped right now as to how to do this because the way I've been told to do so does not work 100% of the times. Basically, I'm trying to create a random tree generator. This tree generator works, except that if I rotate the "Earth" (For now it's only being rotated on the X axis), the trees still spawn with a Y coordinate of:

(Above code works fine)

But if I take the "Earth", and rotate it 45 degrees,...

C# - Find Y coordinate given X value and slope (in degrees)