# How do you find the slope for y=3?

use the definition of slope as $m = \frac{\Delta y}{\Delta x} = \frac{{y}_{2} - {y}_{1}}{{x}_{2} - {x}_{1}}$
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:
$m = \frac{3 - 3}{5 - 3} = \frac{0}{2} = 0$