$\begin{align}  & \angle EOB=\angle ABO=\alpha \text{       }\!\!\{\!\!\text{ alternate angles }\!\!\}\!\!\text{ } \\ & \angle EOH=\angle GHO=\gamma \text{      }\!\!\{\!\!\text{ alternate angle }\!\!\}\!\!\text{ } \\ & \text{At point O} \\ & \angle EOB+\angle BOH+\angle EOH={{360}^{\text{o}}}\text{   }\!\!\{\!\!\text{ sum of angles at a point }\!\!\}\!\!\text{ } \\ & \therefore \alpha +\beta +\gamma ={{360}^{\text{o}}} \\\end{align}$