The angle x is supplementary to the angle 135°
This mean that x + 135° = 180°
Then, we have x = 180° - 135° = 45°
Since y, x and 63° are internal angles of a same triangle, their sum must equal 180°:
x + y + 63° = 180°
45° + y + 63° = 180°
y = 180° - 45° - 63°
y = 72°