Express coordinate z of a point on cycle accoring to angle from (0,0,0)

Question

Hello! The angle of the cycle $a$ is $45^\circ$ towards the $xy$-plane. The center of the cycle is the point $(0,0,0)$. I want to calculate any point on the cycle that has the coordinate $z$ expressed according to angle $b$. An equation that finds the coordinate z according to the angle b.