Explanation:
We have two triangles.
The sum of the angles of a triangle is equal to 180°.
So 180=99+31+y - > y=50°.
Also since we have two parallel lines x and 31° are alternate angles so they are equal. The same goes for the angle nnext to the 31° one and y.
So for the left triangle we have that z+31+50=180 - > z=99