Answer:
y=42
Explanation:
The sum of the angles of a triangle add to 180
y+ 48+ 90 = 180
The third angle is 90 because supplementary angles add to 180 and the other side is a 90 degree angle, which makes that angle equal to 90
y+ 48+ 90 = 180
Combine like terms
y +138 = 180
Subtract 138 from each side
y+138-138 = 180-138
y =42