Question

How do you find the slope for y=3?

Answer 1

You can either:
recognize that this is a horizontal line that will have slope equal to zero;
or:
use the definition of slope as `m=(Deltay)/(Deltax)=(y_2-y_1)/(x_2-x_1)`
choose two values for `x`, say, `x=3` and `x=5` the values of `y` will be, in both cases, `y=3` (unaffected by the change of `x`) so you get:
`m=(3-3)/(5-3)=0/2=0`