Question

What is the slope of a line that is perpendicular to the line y = 1?

Answer 1

Y = 1
Using y = mx + c.
Compare to y = 1,    y = 0x + 1 ,
We can see that the slope m = 0 and the vertical intercept, c = 1.
For the line perpendicular to  y = 1
Condition for perpendicularity  m₁m₂ = -1
m₁ = 0,  m₂ = ?
0*m₂ = -1
m₂ = -1/0 = Negative Infinite or Infinite 
Slope of line perpendicular to y = 1, is  = Infinite. 

Answer 2

One over zero because you start at zero and go up one