Answer:
y=42 degrees.
Explanation:
The sum of angles in a triangle = 180 degrees.
One angle in that small triangle is y, one is 48, and one is unknown.
The sum of angles on one line is 180 degrees.
One of the angles on the line that unknown angle lies on is 90 degrees, so the unknown angle is 90 degrees, too.
90 + 48 = 138
180 - 138 = 42.