Hi,
According to the rules of these forums, we cannot give you the full solution to a homework, you ve got to show us your attempt and then we can figure out your mistakes or give you hints towards the solution.
This problem looks like it can be solved by forming various equations containing angles, all such equations sourcing from the fact that the sum of the angles of a triangle equal to 180 degrees. And also it is given that OA=OC hence the angles A and C are equal.
EDIT: I worked through the problem and i seem to get ##z=x+\frac{y}{2}##...