The unknown (a) is 60° so if you add each angle of this triangle you would get 175° total.
I got my answer by adding 60°+35°+80° and ended up with 175°. In quick math, you can just add 6+8=14 and add the zero so it would be 140 then add 35 which basically you just need to add 3+4=7 then add the zero once again making the answer a 170 and then just add the leftover 5 you got from the 35.