Lines perpendicular to the x-axis are vertical and have no slope,
however they do have equations of the form 

      x = k

The right side of

ax + by = 3

is 3

So if we take k = 3, we would have

x = 3

That's the same as

1x = 3

and also the same as

1x + 0y = 3

ax + by = 3

Ao a = 1 and b = 0

Edwin