We use two angle sums:
---The sum of the angles of a straight line is 180 degrees.
---The sum of the angles inside a triangle is 180 degrees
There are two triangles present. The smallest (left) triangle has a right angle at the top (straight line angle sum). So in that triangle, we know that y + 48 + 90 = 180. Now we move to algebra class.
y + 48 + 90 = 180.
y + 138 = 180 add common terms on the left
y = 42 subtract 138 on both sides
Thus, y is 42 degrees.