The x values are the same for both green dots, so you are looking at a simplified version of the distance formula.
d = sqrt( (x2 - x1)^2 + (y2 - y1)^2 )
Since x1 = x2 = 5, the first set of brackets = (5 - 5) = 0
d = sqrt( y2 - y1)^2 )
d = y2 - y1
y2 = 5
y1 = - 1
d = 5 - - 1
d = 6 Answer