Hi there I hope you are having a great day :) I am pretty sure that you do 280 degrees around angle so i would say you would add 63 + 73 + 83 = 219 then you would take away it 280 - 219 = 61 so y must equal to 61 this is because we can see a z shape and a z shape adds up to 280.
Hopefully that helps you.
