You have no number values there; the best I can do is tell you that c = 360-a-b-d. Since b and d are the same measure, we can rename d as b, so your equation is c=360-a-2b. But since a and c are the same, rename a as c to get an equation of c=360-c-2b. Now add c to both sides to give you 2c=360-2b and c=180-b. Can't do anything else with no values.