Since the mid line is the perpendicular bisector:
Hence angle on the right is also y.
y + 55 + 90 = 180 sum of angles in a triangle.
y + 145 = 180
y = 180 - 145
y = 35
y = 35°
We can go ahead to show that the two angles are congruent ASA
Therefore:
2x + 1 = 13
2x = 13 - 1
2x = 12
x = 12/2
x = 6
x = 6 and y = 35°
Hope this explains it.