y is an exterior angle of the triangle.

An exterior angle of a triangle is the sum of 
the two remote interior angles, that is, the 
two angles that are farthest away from the 
exterior angle y.

80° and 50° are the two remote interior
angles of the triangle because they are the ones
farthest away from y. therefore y is the sum of 
them, and

y = 80° + 50°

Edwin