What does y= ? "for a(1,3) and b(1,5) what is the equation of the perpendicular bisector of ab?

We have that
A(1,3) and B(1,5)

we know that
the equation of the perpendicular bisector of AB  go through the midpoint of AB
so

step 1
find the slope m AB
m=(y2-y1)/(x2-x1)----> m=(5-3)/(1-1)-----> is not defined
because is a vertical line
hence 
the perpendicular bisector of AB is a horizontal line

step 2
find the midpoint AB
Mx=(x1+x2)/2-----> Mx=(1+1)/2----> Mx=1
My=(y1+y2)/2-----> My=(5+3)/2----> My=4
the midpoint AB=(1,4)

the equation of the perpendicular bisector of AB is
y=4

the answer is
y=4

see the attached figure