The sum of all angle measurements in a triangle is 180 degrees. Since all of the angles in this triangle are the same measurement, you can divide 180 evenly among the three angles (one of which is y). 180/3 = 60.
That's an equilateral triangle, meaning all the sides and angles will equal the same. Altogether the measurements for a triangle should always equal 180, so 60 would be the answer because 60 times 3 is 180
