In a triangle, the sum of the angles is always 180 degrees. In the given diagram, we can see that the angles x, y, and z form a triangle. Therefore, we have: x + y + z = 180o Rearranging this equation, we get: x + y = 180o - z Hence, the value of x + y in terms of z is 180o - z. Therefore, the answer is (a) 180o - z.