As the given two points lie on Y-axis distance between them is
|y2 - y1| = |-+| = || = units
Let (0,y1) and (0,y2) be any two points on Y-axis, then distance between them
= √((0 - 0)2 + (y2 - y1)2)
= √(0+(y2 - y1)2)
= √(y2 - y1)2
= |y2 - y1|
[distance cant be negative]