It is a little long to write so I give you a hint and when you're stuck ask again.
The hint is :
Consider the isosceles triangle ACO. Say its identic angles are ... x (or any variable you want). Then start expressing all the angles with respect to x. You can start by the one between the 60 degree angle and the 20 degree angle. It will give you 120 - 2x. Keep doing that and you will find that supplementary angle of alpha is 40. It follows that alpha is 140 degree (if I made no mistakes )